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Call No. 5^ . Accession No. QJ, 33, g-gf 



This book should be returned on or before the date 
last marked below. 

Natural History of 



The Place of 
Birds in Nature 


Birds appeal to a l;irc number of people; some study birds, some 
watch birds, some merely like birds. The interest may be simple 
perhaps only an easy chair before a window lacing a feeding* station 
or a bird bath. ^ et it may lead to profound research deep and 
difficult investigations in the fundamentals of science. It may be pro- 
fessional, though it usually is not (at least among Fnglish-spcakmg 
people). Interest in birds may entice one person into a nearby wood- 
lot; it may send another into distant lands and remote places. Through- 
out the whole gamut of bird study, however, runs the thread of kinship 
interest in birds simplv because people like them. No other reason 
really is needed. Save only for the utilitarian sciences, such as medi- 
cine, engineering, or the like, the number of people studying birds 
and the fund of knowledge consequently built up exceeds that of any 
other comparable field of science. 

The appeal of birds to the intellectual side of man seems to be 
rather recent. The Stone Age artist who painted a bird on the walls 
of a cavern or who carved a bird in mastodon ivory is a kindred 
spirit to the bard whose larks sang at heaven's gate and to the poet 
whose sandpiper flitted along the beach under sullen clouds that 
scudded, black and swift, across the sky. For many a generation, birds 
were left largely to the artist, the poet, and the bard (and of course to 
the hunter). But with the ascendancy of knowledge over superstition 
and curiosity over compliance, men of facts and reason moved forward 
in bird study as in so many other fields. Yet the innate lure to the 


man of esthetics still supplies much of the stimulation that stirs the 
man of science. And for this, science may well be thankful. 

The Lure of Nature. Though we may be sure that the appeal of 
nature to man's senses is far from a new discovery, succumbing to it 
seems clearly more characteristic of today than of any time in the 
past. The reasons seem associated largely with leisure time and the 
capacity to get outdoors along with the means with which to do so. 
Fundamentally (though man may be too vain to admit it!), the devel- 
opment of cheap ways of substituting mechanical power for human 
muscle surely is the chief single factor making possible the more exten- 
sive study of birds. A people who spend most of their days grubbing 
for the necessities of life clearly will have few opportunities for in- 
dulging in bird study or any other lure of nature. Only directly utili- 
tarian undertakings occur when people must live only to survive. 

Scholars debate whether people are interested in natural pursuits 
because it satisfies some ancient, atavistic hunting instinct inherited 
from the remote past. Philosophers write of the joy and peace of 
nature. Yet whatever it may be, the woods and fields and mountains 
lure more and more people to them each year. Bird life is one of the 
luring attractions, and bird study shows many signs of continuing to 
increase in the foreseeable future. Surely, there arc no indications that 
it will decline. 

Attractiveness of Bird Study. To those men and women looking 
for an outlet for their intellectual resources (especially an outlet 
with an esthetic appeal) when their everyday life fails to stimulate 
to fullest intellectual capacity or when the stimulation is so great as 
to generate tension, bird study offers a solid yet relaxing avocation. 
Birds are living things: they have charm, beauty, and the spirit of life. 
They may be found nearly everywhere, at all seasons of the year, in 
all kinds of weather. They may be studied in the neighborhood of the 
home or in the remoteness of distant places. Bird study may be a 
solitary pursuit for those who prefer to be alone; it may be group 
activity for more gregarious spirits. One may study a facet of his own 
choosing; or he may participate in joint efforts toward a common 
task (Fig. 1-1). 

Whatever may be one's talents, training, or experience in vocational 
life, often as not it may be applied avocationally to bird study. Birds 
do about every living thing that man does whether it be singing, 
eating, or just getting sick and some things that modern man cannot 
like flying without mechanical contraption, or living permanently 
and unaided off the countryside. Hence, one studying birds has 
almost a limitless field before him. 


rig. 1*1. Bird study may be a solitary pursuit for those 'who prefer to 
be alone or a group activity for others. 



The observer of today sees birds that have developed from a long 
line of continuing evolution, a long series of events in which the steps 
are unlabeled. The fossil evidence reaching back to Jurassic times, 
some 150,000,000 years ago (see Geological Time Scale, page 10), 
indicates clearly that both birds and mammals evolved from reptilian 
ancestors. The evidence indicates, as one should properly suppose, 
that modern reptiles have changed less during this period than have 
mammals and birds. 

Many factors operate to obscure our knowledge of the ancestry of 
birds. There are many gaps in the fossil record. Bird bones are small 
and delicate and arc therefore less likely to be preserved in fossilizing 
sediments than the heavy bones of other animals. The fact that birds 
fly makes them less likely to die where their remains might be en- 
tombed. We must remember also that birds arc largely land forms 
today and probably were so in the past as well. Obviously, land 
animals are less likely to be covered by fossil-forming muds than are 
water animals. 

Ancestry of Birds. Fig. 1-2 presents a postulated ancestral line 
of the bird and indicates the general relationship of birds to mammals 
and other vertebrates, particularly to the Mesozoic reptiles. Birds arc 
believed to be derived from tbccodonts, commonly called ruling rep- 
tiles. The arcbosaurs, the general group to which the ruling reptiles 
belonged, were derived from stew reptiles known as cotylosaurs. The 
cotylosaurs are the earliest known reptilian descendants of the am- 
phibians, which in turn arose from the fish (Fig. 1-3). Mammal-like 
reptiles as well as turtles seem to have split off from the cotylosaur 
stem sometime before the rise of the ruling reptiles. The evidence 
indicates also that lizards and snakes, as well as Spbciwdoii, the curious 
living reptilian relic of New /caland, branched off earlier. 

The ancestors of the higher archosaurs appear to have adopted a 
tendency for a bipedal mode of life, and from ancestral archosaurs 
sprang the dominant reptiles of the Mcsozoic. Among these were three 
specialized groups: the Ftcrosauria, or flying reptiles; the Ornithischia, 
or "bird-pelvis" dinosaurs; and the Saurischia, or "reptilian-pelvis" 
dinosaurs. Sometime during this period, the Crocodilia branched off 
from the generalized line of ruling reptiles that also gave rise to birds. 

It seems most probable that the primitive ancestral birds arose from 
a generalized archosaur that was somewhat more primitive than 
Pterosauria, Ornithischia, and Saurischia. It is certain that the bird did 
not arise from the flying reptiles of the Mesozoic (Fig. 1-4). The 







Wi ' -V 

t Xpr/ 









Fig. l2. Postulated ancestry of birds. The tbccodont illustrated 
(Ornithosuchus) Is generally used to show a prh/ritive ruling reptile, but 
the counnoii ancestor of birds and later ruling reptiles probably had better- 
developed forelhtibs than those shown. (Prepared by Avne Hinshaiv 


Fig. I 3. A suggested evolutionary line of the bird from the fish. (By 
permission ]rom Evolution Emerging, by William K. Gregory, p. 546. 
Copyright, 19)1, The Macvjillan Co., New Yoik.) 

Fig. I *4. Restoration of a pterodactyl, a ftyiug Mcsozoic reptile not , 
ancestor of the bird. 


flying reptiles formed a different and most distantly related group. 
They lived and flew; otherwise, they had little in common with birds. 
The stem reptiles, from which the ruling reptiles came, probably arose 
some time in the Paleozoic, perhaps in the Permian. The archosaurs 
themselves seem to have arisen early in the Mesozoie, during which 
period the primitive bird line as well as Spbenodon may have been 

Fig. I '5. Restoration o] a bipedal dinosaur (Ornithomimus). 

Paleontologists raise some rather fatal objections to the idea that 
birds came from a bipedal, dinosaur-like Ornithischian (Figs. 1'2, 
1-5). One objection is that a bipedal life naturally results in a reduc- 
tion of the forearm in favor of the hind legs. This can be seen today 
in the kangaroo, jumping mouse, and many other animals as well as 
in man himself. If nature abhors a vacuum, it might also be said that 
evolution dislikes to reverse itself, to backtrack over its trail. If birds 
developed from a bipedal ancestor, the dwindling forearms would 
have had to increase in size and importance all over again. Thus evolu- 



tion would tend to reverse its trend. This objection has been met in 
part by assuming that the bipedal ancestor lived in the trees (hence, 
led an arboreal life) and therefore always had well developed forearms. 
The similarities of the skeleton are said by some to indicate descent. 
But more common agreement holds that any similarities might well 
result from similarities of life or from retention of some ancestral 
traits not particularly handicapping. Most students of the subject feel 
confident that, with few exceptions, the more specialized reptiles 
perished in the upheavals and competitions of the past. Birds thereby 
arose from a more generalized type of reptile that lived in relative 
obscurity during the days of dominance by the higher archosaurs. 

Mesozoic Birds. Bavarian slate carvers discovered in 1861 the 
earliest fossil clearly of the bird line, which now goes by the scientific 
name of Archaeopteryx inacrura. The slate entombing the fossil be- 
longs to the Upper Jurassic period of the Mesozoic era (see Geologic 
Time Scale) . Although at first paleontologists considered it as possibly 
a birdlike reptile rather than a true bird, general agreement now con- 
Table I- 1 
Geologic Time Scale 


Period Millions of 
and Years Ago 
Epoch to Beginning 

Duration Characteristic Events 
(Millions Birds and 
of Years) Other Animals 



Man, modern birds, 

Age of Mam- 


and mammals 

mals and Birds 




Ice age, modern birds. 


Many birds became 











Rise and development 

Eocene * 



of modern birds 






Age of Rep- 









First mammals 





Earliest true reptiles 

Age of Prim- 




Age of amphibians 

itive Life 




True fishes 




Armored fishes 




First fishes 




Invertebrates only 


Little record of life 

Age of Uni- 
cellular Life 


* The early part of the Eocene is sometimes called "Paleocene." 


siders it to be a true bird, albeit a reptile-like one. The impressions in 
the slate beds show that the bird did have feathers; obviously, if it had 
feathers, it was a bird. A neat point could be made, of course, on 
when a feather-like structure becomes a true feather, but it seems 
clear that Archaeopteryx possessed true feathers. Archaeopteryx was 
about the size of a Crow, and the skeleton has been well preserved 
for a bird of this size. Feather impressions of the tail and the wing 
show plainly; the bones show that the creature had a long tail with 
feathers sticking out from each side. The completeness of the wing 
bones also shows a very great advance of evolution; although we may 
not consider Archaeopteryx as a flying bird after the modern manner, 
it certainly should be considered an aerial one. 

Fig. I 6. Restoration of Archaeopteryx. 

Sixteen years later fortune smiled again upon the field of orni- 
thology when in 1877 a second and more complete skeleton of a 
Mesozoic bird was discovered in the Bavarian slate beds (Upper 
Jurassic). This bird, about the size of a pigeon, received the name 
Archaeornis (now Archaeopteryx) siemensi (Fig. 1-6). The 
feathers on the body show clearly, as do those of the wings and tail. 
In fact, the feathers are so complete that some even show the pres- 
ence of barbs, which must have taken a considerable period of evolu- 
tion for development and luck for preservation. There can be little 
doubt but that this creature flew, even though it possessed teeth and 
other reptilian traits. The sclerotic ring of the eye was as well devel- 


oped as it is in many contemporary reptiles and in many birds of 
today (see Fig. 4-20). Thirteen pairs of cone-shaped teeth_ lined 
the maxillary and premaxillary bones, where they grew in separate 
sockets~~as 'in reptiles. The ribs of Archaeorms (Achaeopteryx) 
lacked the"uncinate processes found in modern birds and had a round 
and slender shape rather than the flattened shape of modern birds. 
We must not assume from this that uncinate processes were not 
actually present, for they may have been of cartilage. Archaeorms 
(Archaeopteryx) also possessed abdominal ribs, now lacking in birds 
but still present in reptiles. 

A long evolutionary history no doubt lay behind these fossils. The 
modification of the finger bones for flight certainly must have required 
a long time, not to mention feather perfection. The substantial 
humerus, for example, has large ridges for the attachment of flight 
muscles, and from this alone we may conclude very properly that the 
wings were rather powerful locomotor organs. The radius and ulna 
were unmodified for the most part, but the ulna possessed a large head 
for muscle attachments. The wrist bone and hand bones were present 
and rather unmodified also, but the finger bones were mostly fused 
together. The second, or index finger, seems to be like that in modern 
birds. The third finger was fairly well developed, but the bones of 
the fourth and fifth fingers had already disappeared. The first finger 
(thumb or pollex) was but a rudiment even in that early day. 

The muscles of the forelimb no doubt had gone through many 
evolutionary changes. The upper arm muscles apparently were large 
and powerful, though the hand muscles were almost nonexistent, 
while those of the lower limbs were much reduced and rather weak 
as in modern birds. The flight muscles (pectoral) were evidently 
highly developed and massive in order to operate the arm, a conclu- 
sion which has been reached (along with others) through examination 
of the bony structure. 

Cretaceous Birds. During the Cretaceous period lived many kinds 
of reptiles, on land, in the sea, and in the air. Some of the great flying 
reptiles had wingspreads of 25 feet (which makes them the largest fly- 
ing structures before the airplane), and some were small like Archae- 
opteryx and probably even smaller. 

In the Cretaceous deposits of Kansas and Colorado occur the re- 
mains of a bird that descended to the water and undertook a marine 
life at the expense of its aerial one. Hesperornis lost most of the 
bones of the wing but had well-developed feet placed far back on the 
body for strong pushing strokes and surely was a swimmer and diver 
of superior ability. Teeth still filled the jaws, but in other ways it was 
clearly avian and shows few primitive characters so evident in earlier 


forms. A bird fossil described as Ichthyornis from the Cretaceous 
marine deposits of Kansas (on the basis of an incomplete skeleton) 
has been shown to have had a jaw agreeing with the jaw of mosasaurs. 
It is concluded therefore that the jaw found with the fossil was not 
that of a bird (Gregory, 1952). 

It seems clear enough, even with the scanty information available 
through the fossil record, that the bird of the Mesozoic was indeed a 
bird. Hence, we may conclude that any further development would 
be more properly the evolution of the bird rather than its origin. The 
study of birds from the Mesozoic onward becomes the study of crea- 
tures already birds. The birds of the Mesozoic may therefore be 
considered as true birds but with strong reptilian leanings; those of the 
Cenozoic were essentially modern birds. 


Because birds are animals, they must be examined and considered 
with respect to other animals (though by themselves they form a 
well-marked group, which often enough is sufficiently satisfying to 
students of ornithology). Their relatives are more than just the rep- 
tiles from which their line came so long ago in human concept but so 
recently in the light of geological time. 

Biologists recognize two major divisions of living things and have 
designated them as the PLANT KINGDOM and the ANIMAL KINGDOM.* 
While it is simple to classify common organisms as either plants or 
animals, such simplicity fails in border-line cases. In a sense, this should 
not be surprising, for there is no reason to believe other than that all 
life evolved from the same source and that a division however in- 
distinct occurred somewhere along the line. 

Phylum. The living things of the ANIMAL KINGDOM have been 
divided into a number of great groups, called phyla, having distinct 
enough characteristics so as to indicate affinity. Zoologists do not 
always agree upon the characteristics that indicate relationships. Nor 
do they always agree upon what are or are not phyla. In order that 
birds may be visualized in proper perspective and in order that bird 
students who are not familiar with the various phyla may have their 
names available for ready reference, the twenty-one of more general 
agreement are given below: 

PROTOZOA (Animals of one cell) 

MESOZOA (Parasitic, primitive multicell animals) 

PORIFERA (Sponges and their allies) 

* Some biologists consider Fungi to be a third kingdom. Some also have doubts 
about the classification of bacteria, rickettsia, and viruses in the scheme of two king- 


COELENTERATA (Radial animals with nematocysts; polyps and me- 

CTENOPHORA (Radial animals without nematocysts; comb jelly- 


NEMF.RTINEA (Ncmertine or freshwater flatworms) 

ENTOPROCTA (Minute stalked animals) 

ASCHELMINTHES (Round and horsehair worms) 

ACANTHOCEPHALA (Thorn-headed worms) 

BRYOZOA (Colonial, aquatic organisms) 

PHORONIDEA (Wormlike, marine animals) 

BRACHIOPODA (Marine, shelled animals) 

ECHINODERMATA (Radially symmetrical animals) 

CHAETOGNATHA (Small, aquatic worms) 

MOLLUSCA (Oysters, clams, and their allies) 

ANNELIDA (Segmented worms) 

SIPUNCULOIDEA (Peanut worms) 

PRIAPULOIDEA (Marine organisms) 

ECHIUROIDEA (Marine organisms) 

ARTHROPODA (Jointed-limb animals with exoskeltons) 

CHORDATA (Animals wtih a notochord) 

Chordata. Birds belong to the phylum Chordata, which may be 
separated into several subphyla and classes: 


SUBPHYLUM HEMICHORDATA.* (Marine, wormlike chordates with gill slits) 
SUBPHYLUM TUNICATA. (Marine, larva-like animals covered with a "tunic") 
SUBPHYLUM CEPHALOCHORDATA. (Small fishlike primitive chordates) 
SUBPHYLUM AGNATHA.** (Vertebrates without jaws and usually without 
paired fins or appendages) 

Class Cyclostornata (Lampreys and hag-fishes) 

SUBPHYLUM GNATHOSTOMATA.** (Vertebrates with true jaws and usually 
with paired appendages) 

Class Chondrichthyes (Sharks and rays) 

Class Osteichthyes (Bony fishes) 

Class Amphibia (Amphibians) 

Class Reptilia (Reptiles) 

Class Aves (Birds) 

Class Mammalia (Mammals) 

Characteristics of the Bird. All chordates have a notochord^ It 
appears briefly in the embryo, but serves no longer as'a stiffening rod 
in the adult, as it does in amphioxus and in cyclostomes. It does serve 
however, as the foundation around which the bird backbone forms 
during embryonic development. Chordates all have pharyngeal gill 
slits. They are present in the early bird embryo but develop into 
other structures as the bird grows. All chordates have a nervous sys- 

* Some zoologists call this a phylum. 

** Many zoologists prefer to use the single subphylum VERTEBRATA for these; 
others prefer to use CRANIATA. 



tern dorsal to the^ digestive tract, a characteristic seen in any bird 
dissection (Fig. F7).~The bird has a vertebral column as well as a 
cranium. We can readily understand why tHe bird is placed in the 
Subphylum Vertebrata or Subphylum Craniata, as the choice may be. 
Birds also show the vertebrate or chordate characters of bilateral sym- 
metry, internal metamerism (segmentation), anterior mouth, anJ di- 
vision of the body into bead, trunk, tail, and limbs. 




oil gland 


vas deferens 

muscle' /^T \ ^ QI2zard 

proventnculu's / \ * Q|1 bladde ' 

liver sternum 

Fig. I 7. Internal anatomy of the Domestic Chicken shows the general 
character of the bird anatomy. 

All classes of vertebrates have characters that are designated as 
distinguishing characteristics. Not always are these the exclusive pos- 
session of one class but may be shared with one or more others. The 
nine most commonly used distinguishing characteristics for birds 
(class AVES) may be listed as follows: 

1. Possess feathers 

2. Primarily land dwellers 

3. Possess four limbs 

4. Adapted for aerial life 

5. Hbmoiothermous (warm bloodedness) 

6. Have four-chambered heart 

7. Right aorta only usually persists 

8. One occipital condyle 

9. Lay eggs and care for young 



All birds, even the most aquatic of Grebes, Penguins, and Alba- 
trosses, return to land (or shore) for nesting. The aquatic adaptation 
of such birds is secondary, however, for they must return to land 
during the breeding season. 

The bird shares a bipedal mode of life with the extinct ruling rep- 
tiles (as well as with a few scattered animals, including man). The 
ruling reptiles too had four toes to the foot (and some had three 




Blue Jay 
Bob- white 


I | 

1 1 1 1 

1 1 1 I 



97 98 

99 100 101 102 

103 104 105 106 

107 108 109 110 

Fig. I 8. The body temperature of birds varies from about 104 F. to 
about 110 F., somewhat higher than in mammals. 




& 90 





Bird temperature 

I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 


Fig. 1-9. The newly hatched bird has little temperature control and 
is essentially ''cold-blooded.'" Body temperature becomes established for 
the House Wren on about the ninth day. (Modified by Kendeigh from 
S. Charles Kendeigh and S. Prentiss Baldwin, Physiology of the Tempera- 
ture of Birds, Cleveland Museum of Natural History, Science Publications, 
No. 3, 1932.) 



fingers on the forelimb, also in common with birds). The bare essen- 
tial bracing of any structure is four ways. The_biri therefore Jba& two 
toes set at an angle for side sway, one ahead for forward tip and usu- 
ally one behind for rearward tip. To facilitate walking, the side 
"braces" have moved forward somewhat. The rear toe has diminished 
in importance in some walking (but not perching) birds. 

Warm bloodedness (howoiotherwous, technically, constant tem- 
perature) birds and mammals have in common. Other vertebrates are 
cold blooded (poikilothermous), and their body temperature varies 
with that of the surrounding medium rather than being almost wholly 
independent as in homoiothermous animals. The temperature of 
placental mammals normally ranges between about 96 F. and 103 F., 
according to species, while that of birds ranges between 104 F. and 
110 F. (Figs. 1-8, 1-9). In a sense, mammals are warm-blooded and 
birds are "hot-blooded," but the temperature of a hibernating mammal 
may reach about 36 F. and that of a torpid Nighthawk drop to 80 F. 
In summer a reptile may have a temperature matching that of the air, 

Table 1-2 
Comparative Class Characters 




Four limbs 

Four limbs (front limbs 

Four limbs 


Dry skin 

Dry skin 

Dry skin 




Two aortas present 

Right aorta only usually per- 

Left aorta persists 





Nucleated ovoid corpus- 

Nucleated ovoid corpuscles 

Nonnuclcatcd discoid 



One occipital condyle 
Sac-like lungs 

One occipital condyle 
Fixed lungs 

Two occipital condyles 
Elastic lungs 

Three-chambered heart 

Four-chambered heart 

Four-chambered heart 

(or incompletely four- 

chambered *) 

Egg laying 

Egg laying 

Young born alive (usu- 

Seldom care for young 
Two ovaries functional 
Rudimentary corpus cal- 


Poorly developed cere- 

Care for young 

Right ovary degenerate 

Small corpus callosum 

Well-developed cerebellum 
Oblique septum 

Care for young 

Two ovaries functional 

Well-developed corpus 

Well-developed cerebel- 


Mammary glands 

* The Crocodilia have a four-chambered heart, but a slight mixing of blood may 
occur, so that they should be considered as lacking the completeness of four chambers 
as in birds. 



which may exceed 100 F. Prolonged exposure to a temperature 
much above 112 F., however, may be fatal both to reptiles and to 

Reptiles, Birds, and Mammals Compared. The long time that 
has separated the reptiles, birds, and mammals from each other has 
made for many differences. Yet many things in common have been re- 
tained. A comparison of some characteristics of the three groups 
indicates the relationships of several of the comparative characters 
(Table 1-2). 



- Mammal 



[Ostrich 1:1020] 









Fig. I 1 0. Ratio of heart size to body size in vertebrates. Heart-body 
ratios of birds are indicated by X. (The ratio of heart size to body size 
in the Ostrich is of too great a magnitude to be shown in this figure.) 





Fig. I 1 1 . The right aortic arch is the one that usually remains in the 
adult bird (and the left in the adult mammal) though the reptile has 
both functional. 



The greatly modified wings of today seem unique. They have 
three digits remaining, but reduction in the number of digits to three 
occurred among the ancient reptiles. The hollow bones of birds are 
likewise unique among living animals, thTJugK some extinct reptiles 
had them. Sclerotic plates, which strengthen the eye in birds, have 
been found in some reptiles; the fossil remains of Mesozoic ichthyo- 
saurs and pterosaurs show them. 

The four-chambered heart of the bird is comparable to that of the 
mammal and is of about the same or greater proportionate size (Fig. 
1-10). Usually little or no trace of the left aortic arch remains in 
the adult bird (Fig. Ml). 

Replacement of teeth with a horny beak characterizes modern 
birds, but extinct birds had teeth and modern turtles have beaks. At 
the opposite end of the body, the tail has been much reduced. No 
reptile and few mammals have so great a reduction. The hind limb 
of the bird has an intratarsal or mesotarsal joint in common with some 
reptiles (page 57). Even the outer surface of the lower leg and foot 
of the bird still bears scales, which is probably an ancient reptilian 
trait still efficient in protecting the foot with a light-weight covering 
(Fig. 1-12). 

Fig. 1-12. Bird feet are variously adapted for various types of life, 
(a) Yellow-legs, (b) Pheasant, (c) Flicker, (d) Passerine, (e) Ostrich, 
(f) Hawk. 



(c) (d) 

Fig. 1-13. Skulls of (a) modern reptile, (b) ancient toothed bird, (c) 
modern bird, and (d) modern mammal. 

The skull shows many special variations distinctly unlike those of 
any reptile, but the skull also shows many retentions of reptilian 
form. The several bones forming the lower jaw are rather siifiilar to 
those of reptiles (Fig. 1-13), though even in early birds, the jaw 
seems to have differed. There have been marked changes in th.e brain, 
especially in the increase of the cerebrum and cerebellum (Fig. 4-18). 

Adaptations for Flight. The use of flight, the most outstanding 
single controlling characteristic of Eir3s, dominates the whole nature 
of the creature. As in an airplane, the framework plays a leading 
role. But the framework of the terrestrial ancestor, whatever it may 
have been, has had to be drastically adjusted to fit it for flight. 

Because all soft parts are fastened to the skeleton, the bony frame- 
work forms a good place for beginning a general discussion of flight 
adjustments. The entire skeleton has become compact; much of this 
has been brought about by fusion of parts, particularly of the bony 
basket itself, and consequent reduction in bulk through elimination 
of connective tissue, ligaments, muscle, and other soft parts. Fusion 
makes for rigidity; in consequence, the bird body itself has little move- 
ment. Reduction of weight in proportion to motive power increases 
flight capacity, and birds carrying less ''dead weight" have a distinct 
flying advantage over others. Increase in use of hollow bones and 
shifting of weight from extremities accompany marked flight im- 
provement. This is especially true in respect to the wings and legs 
that have to be swung in an arc like a pendulum, or otherwise moved 
when suspended (Fig. 1-14). 

Allied with skeletal compactness is the general streamlined shape 
of the body. Full streamlining requires a blunt forward end tapering 
to a pointed stern. The faster flyers are much better streamlined than 


Fig. I 14. Comparison of the wings of a flying reptile, bird, and bat. 
The numbers indicate which digits are present in the wings. 

Fig. I 15. Varying degrees of streamlining in birds. Left to right: 
Duck, Shearwater, Heron, Falcon, Swift, Booby. 

the slower ones (Fig. 1-15). The Chimney Swifts show this especially 
well; they have a short, rather large and blunt head. The feathers 
of the neck carry the streamlining into the shoulders and breast, the 
greatest circumference of the body. From the shoulders rearward, 
the body tapers to a short tail. Even the feet have been reduced in 
size and strength, and the pelvic girdle along with them, so that the 
feet fit into the streamlined feather mass when drawn up. In line with 
this reduction in foot power, the Chimney Swift can cling well only 
on vertical surfaces where its tail can help the weak feet. 

The penetration of so many parts of the body by air sacs seems to 
help somewhat in flight efficiency. But air sacs function chiefly for 
cooling. Air uses less weight to carry heat from the interior than does 
blood. The heat is expelled directly to the exterior rather than indi- 
rectly by conduction, radiation, or evaporation. In a sense, the bird 
has an "air-cooled" body in marked contrast to the "liquid-cooled" 
body of man. 

Another trend in the bird world, again in line with aerial life, is 
the movement of weight to the interior, so that the center of gravity 


tends to lie slightly back of the wings. The shift in emphasis from 
muscles in the wings to the great pectoral flight muscles m the body 
exemplifies this (see Fig. 4-2). The wing is raised by the inner breast 
muscles below the wings rather than by topheavy back muscles. The 
heavy teeth of other vertebrates have been replaced by a light, horny 
beak. The grinding function has been taken over in those species 
needing it by a muscular gizzard. The weight of the gizzard may be 
no less and perhaps actually greater than some jaw muscles, bones, 
and teeth for mastication. But the additional neck structure needed to 
support a heavy chewing jaw is not needed, and thus the gizzard setup 
probably weighs no more than a jaw and its accessories. But it is in 
the middle of the body instead of at the end of a column. The reduc- 
tion in head weight has been paralleled by loss of the heavy reptilian 
tail, which if persisting would require counterbalancing by shifting 
weight forward of the wings or shifting the wings backward. 

The intestines, especially the large intestines, have also been re- 
duced in bulk. In fact, the entire internal organs evidently have been 
abbreviated when possible to do so without materially lowering physi- 
ological efficiency. It is likely that one of the functions of the high 
body temperature of the bird may be to increase energy output with 
reduction of weight. 

The inability of the bird to use the forelimbs as arms and hands 
would be a handicap in feeding, nest construction, and the like were 
it not that a flexible neck has evolved to make it possible for the beak 
to take over such tasks as are usually performed by "hands." The 
development of a flexible neck releases the forelimbs for flying and 
puts the forceps that take up food at the front, along with the mouth. 

We can safely make the general statement that the skeletal struc- 
ture of the bird uses practically all the sound engineering principles 
for combining strength, rigidity, and economy of material. Among 
these may be listed: 

T-beam construction of sternum (primarily for muscle attachment) 

I-beam construction in many bones 

Tubular bones 

Flattening and overlapping of rib processes for shaping and bracing 

Fusion of parts, like castings 

Fusion, thinning, ossification of brain case for lightness with strength 

Distinguishing Features of the Bird. A summary of the important 
factors and adjustments that distinguish the bird from other animals 
or mark its "high-speed" way of life would include: 

An insulating layer of feathers 

Primary adaptation for land life 

Absence of skin glands except uropygial or wattle glands 


Forelimbs modified into wings with three digits (Fig. 1 14; some ancient 
reptiles had three digits) 

Homoiothermous (as arc mammals) 

High body temperature 

Four-chambered heart (as also in Crocodilia and mammals) 
, Large heart (proportionately the largest of any organism) 

Large blood vessels (proportionately the largest capacity of any organism) 

Single, right aorta 

Lung air sacs (in unrelated chameleons also) 

Excurrent and incurrent bronchi 

Hollow bones (Some unrelated pterodactyls had them.) 

Metanephric kidney (in reptiles and mammals also) 

No urinary bladder 

Single left ovary and oviduct (Some Hawks may have two ovaries nearly 
equally developed.) 

Uncinate processes on ribs (found in the reptilian Sphe-nodon also) 

Ey^s highly specialized (perhaps no more than many other animals) 

Greatly elongated, tetraradiate pelvis (as in some reptiles) 

Hind limb with intratarsal joint (as in some reptiles) 

Extensive fusion of trunk vertebrae 

Fusion of vertebrae with pectoral girdle 

Greatly reduced tail 

Lack of teeth which are replaced by horny beak (Some extinct birds had 
teeth and modern turtle has a beak.) 

Egg laying (as in reptiles and mammalian Prototheria) 

Eggs incubated by heat of body (but egg and embryonic membranes es- 
sentially reptilian) 

Parental care of young highly developed 

Scales on lower legs and feet 

Diapsid skull (as in most reptiles) 

Prelachryjiial fossae (as in some reptiles) 

Cloaca present 

Several bones of lower jaw as in reptiles 

Single occipital condyle present 

Bipedal (as in some reptiles and as in man) 

Four limbs (as in amphibians, reptiles, and mammals) 

Adapted for aerial life 

Communication by sound and song 


GREGORY, WILLIAM K., Evolution Emerging. New York: The Macmillan Co., 1951. 
HEILMANN, GERHARD, The Origin of Birds. New York: Applcton-Century-Crofts., 

Inc., 1927. 
HYMAN, LIBBIR HENRIETTA, Comparative Vertebrate Anatomy. Chicago: University 

of Chicago Press, 1942. 

NEWMAN, H. H., The Phylum Chordata. New York: The Macmillan Co., 1939. 
*ROMER, A. S., The Vertebrate Body. Philadelphia: W. B. Saunders Co., 1950. 
*STORER, TRACY I., General Zoology. New York: McGraw-Hill Book Co., Inc., 1951. 
* YOUNG, J. Z., The Life of Vertebrates. Oxford: Clarendon Press, 1950. 

* Books especially useful or interesting to people interested in birds are designated 
by an asterisk (*). 


Classification and 

The mind of man cannot embrace the vast array of separate items 
of knowledge available to it. Naming and arranging objects is a 
fundamental human trait, not just an invention of naturalists. Primi- 
tive man thousands of years ago doubtless named things and grouped 
like ones together in his mind, which thus made for him a crude sys- 
tem of classification and nomenclature. But no matter how crude, 
classification and nomenclature are systems of organization. The 
brain can comprehend and remember an amazing mass of information 
that has been systematically arranged. 


In the science of classification, similar animals (or plants in the 
PLANT KINGDOM) are arranged together into related groups. But 
before they can be so arranged, their relationships to each other and 
to other groups must be determined. In addition to being given a 
name, their salient characters must be listed so that another may recog- 
nize and identify them. Sorting (classification),nammfi ('nomencla- 
ture^ and describing (description) tOfferiiei' destitute classical tax- 
owomy^. But the very great accumulation of information on the 
biology of living birds has brought about a marked change of emphasis 
in bird taxonomy. While earlier it concerned itself chiefly with the 
study of relationships as shown by bird skins and anatomy, some tax- 
onomists now recognize that any bird characteristic, whether of be- 
havior, physiology, ecology, or structure, may show relationships.* 
Hence, some more advanced taxonomy may more correctly be con- 



sidered as having become comparative bird biology in a synthesis of 
all ornithological knowledge around the theme of relationship. In a 
somewhat similar way, this book is one of comparative bird biology, 
but its theme is the life of the living bird. , 

Relationship. The fundamentals of relationship among birds are 
similar to those in man's own family connections i.e., siblings are 
more closely related than first cousins. Ifce determinant is distance 
removed \rorn the coitniron anr,e$tf>r. Thus siblings are one step re- 
moved, first cousins are two steps removed, and so on. In actual prac- 
tice, the taxonomist may judge relationship only on the basis of the 
birds as they arc today. The connecting links have long since disap- 
peared, and the continuous record has been broken except as we are 
able to determine it directly from fossils (Fig. 2-1) and indirectly 


Fig. 2*1. Exhaustive study of fossil remains tells paleontologists some- 
thing about the bird. Comparison of the skeletal structure of the fossil 
with known birds gives additional ideas about the probable life of the 
ancient bird, (a) Distal end of ulna of Palaeoborus rosatus of the Mio- 
cene, related to Old World Vultures, (b) Tarsowetatarsits of Paranyroca 
inagna from the Miocene, a distant and primitive relative of the Anatidae 
about the size of a Swan. These are type specimens. (After Alden H. 
Miller and Lawrence V. Cowpton, "Two Fossil Birds from the Lower 
Miocene of South Dakota,'" Condor, 41(1939):155). 

from modern or recent birds and their attributes. Morphological 
characters are used to indicate major relationships, although other 
characters may be used as supporting evidence. The more like the 
ancient, fossil ones a living bird is, the more primitive it is said to be. 
In addition to the evolved character of the bird itself (Chapter 9), 
among the many things that may indicate relationship are such things 
as parasites; evidently parasites stayed with their host through its evo- 
lutionary history and may show relationships not readily apparent in 
the bird itself (page 405). 

Species. The smallest mqjor unit- rr g"i^ hy \*n i-ovonomicrg 
isjhc species (though there may he subunits within a species). While 
we cannot set up any criterion for a species so definitive as to leave 
no room for doubt, it does seem possible to have a practical definition. 
A species may be defined as a population of * 


among themselves but are reproductively isolated fro?u others^ Thus 
the Crow of North America is a species. The Raven that looks just 
as black is a different species, for Ravens and Crows do not cross in 
nature. They are reproductively isolated, but the exact mechanism 
for this is immaterial to our recognition of the fact of isolation. A 
familiar example or two may clarify this concept further. 

The Downy and Hairy Woodpeckers of North America bear 
striking resemblances to each other. The color pattern of one, for 
example, duplicates that of the other except for a few minor differ- 
ences, as in the presence or absence of bars on the tail. There can be 
little room for doubt that these two species are closely related to each 
other. Yet no case has been found where these two birds clearly 
cross, even though they occupy much the same range over the ,con- 
tinent. To such birds has been applied the term ^^{^ **er>i** 
(see also allopatric species, Chapter 10). 

The Peregrine Falcon inhabits much of the Northern Hemisphere, 
both Old World and New World, wherever conditions are suitable 
for it. The Falcons of America are geographically separated by water 
from those of the Old World; yet the birds are the same, for they 
grade completely into each other from one end of the range to the 
other. But in western America, the Prairie Falcon differs somewhat 
from the Peregrine and no interbreeding occurs between them. The 
Pergerine and Prairie Falcons are thus reproductively isolated, even 
though not now geographically isolated. Hence, they constitute two 
different though closely related species. 

Subspecies. Birds of wide distribution and some of rather re- 
stricted distribution sometimes break up into geographic groups called 
subspecies. These may be referred to sometimes also as races orfowtrs. 
In a sense, subspecies mark the plasticity of a species in adapting itself 
to ecological conditions. But the life habits and characteristics of the 
species affect this plasticity. A resident species, for example, tends to 
have more subspecies than a migratory one. The Canada Jay illus- 
trates well the break-up of a wide-ranging species (nearly 2,500,000 
square miles) into many races (Fig. 2-2). The ten subspecies illus- 
trated inhabit separate ranges having areas of about the following 
square miles: 

fumifrons 1,002,400 obscurus 67,200 

pacificzis 12,320 griseus 117,600 

albescens 851,200 connexus 128,800 

canadensis 815,000 bicolor 156,800 

rathbuni 8,000 capitalis 274,400 

Genus. When one looks at the various species with which he is 
familiar, he notes that several sometimes resemble each other more 




1 fumifrons 

2 pacificus 

3 albescens 

4 conadensis 

5 rathbum 

6 obscurus 

7 gnseus 

8 connexus 
\ 9 

10 copitalis 

Fig. 2*2. Distribution of western races of the Canada Jay (Pcrisoreus 
canadensis). Solid dots show localities of one or more specimens examined 
by the taxonomist in reviewing the status of the subspecies. Circles repre- 
sent other records of occurrence and dots enclosed by circles locate type 
localities. (From John W. Aldrlch, "Relationships of the Canada Jays in 
the Northwest;' Wilson Bulletin, 56(1943):220.) 


than they do others. Anyone familiar with the Lincoln, Swamp, Fox, 
and Song Sparrows of America should be able to see that they resem- 
ble each other more than they do other native Sparrows. Taxono- 
mists place such jjroups of species in divisions, each known as a genus 
(plural generttimA. genus, therefore, is made up of a number of closely 
related specie^ The characters commonly used by avian taxonomists 
in forming a genus are generally plumage pattern and bill shape. 

Family. Many genera in turn bear resemblances to each other that 
set them apart from other genera. Families are formed of such-groups 
of genera. The characters ordinarily used to group genera into fami- 
lies are often bone structure, bill shape, wing formuhu_andLtarsal 
scales^ but they may"lhclude additional characters. 

Almost everyone recognizes the gallinaceous, chicken-like birds as 
being related to each other. Taxonomists have divided these birds into 
several score of genera. Among the gallinaceous birds will be found 
several groups of genera that look and act much alike, and seem more 
closely related among themselves than to other groups. One such 
group of eleven genera bears the mutual common name of "Grouse" 
in recognition of this similarity. They belong to the family Tetra- 
onidae. Other American gallinaceous relatives of Grouse belong to 
Mcleagrididae (Turkey), Phasianidae (Pheasants, Quails), and Cra- 
cidae (Chachalaca, Curassows, Guans). The characters that separate 
the Grouse from the other groups are their mutual similarity of habits, 
appearance, and structure. They are the only gallinaceous birds, for 
example, that have feathered nostrils and feathered tarsi. All of them 
also develop "snowshoes" in winter; their distribution is boreal. It is 
clear that the Grouse descended from a common, Grouse-like ances- 
tor, just as all Phasianidae came from some other ancestor of Pheasant- 
like habits. It is quite likely, however, that Tetraonidae, Phasianidae, 
and other gallinaceous families long ago had a common ancestor. 

Order and Class. ^Various families of birds show mutual similari- 
ties, and they in turn are grouped into orders^tht largest taxonomic 
group within the class*\ Thus, all gallinaceous-turds, regardless of fam- 
ily, belong in the order Galltfonnes. All Hawks, Falcons, and their 
allies belong in the order Falcontformes. Thirty-four such orders are 
currently recognized today; this includes known fossil orders. (We 
have no idea, of course, how many extinct ones there are.) The chief 
characters used in establishing these orders are the shape and propor- 
tions of the bones. 

The Ruffed Grouse of the northern forests is classified as follows: 

Kingdom: Animal 
Phylum: Chordata 


Subphylum: Vertebrata 
Class: Aves 
Order: Galliforrnes 

Family: Tctraonidae (Grouse and Ptarmigan) 
Genus: Bonasa 
Species: wnbellus 

Although the eight groupings shown in the example arc the stand- 
ard ones in animal taxonomy, additional ones are often used for con- 
venience. A family having a large number of genera may have sev- 
eral genera resembling each other more than they resemble the re- 
maining genera. The family may thus be divided into subfamilies. 
Additional groupings may include: 

Subclass Subfamily 

Superorder Subgenus 

Suborder Subspecies 


Scientists have used the Latin and Greek languages for scientific 
names (Latinized) because the grammar and forms of a "dead" lan- 
guage are static and therefore do not change as modern languages do. 
The names of the various orders, families, and other classifications of 
birds have been more or less standardized by the International Rules 
of Zoological Nomenclature. The names of the orders, for example, 
end in -tfonnes (e.g., Galliformes), although some ornithologists still 
use older names which were compiled before an attempt was made to 
bring about more uniformity in scientific nomenclature. The names 
of families end in -idae (e.g., Tetraonidae) and those designating sub- 
class end in -ornithes. In addition, the following endings are used for 
other groupings: -gnathae for supcrorder, -oidea for superfamily, -/ 
or -ae for suborder, and -inae for subfamily. The use of the various 
endings can be illustrated by the complete classification of the Desert 
American Sparrow Falcon in groups, subgroups, and supergroups, 
from class to subspecies: 

Class: Aves (Birds) 
Subclass: Neornithes (True Birds) 
Superorder: Neognathae (Typical Birds) 

Order: Falconiformes (Vultures, Hawks, and Falcons) 
Suborder: Falcones (Hawks, Falcons, and Secretary Birds) 
Superfamily: Falconoidea (Hawks and Falcons) 
Family: Falconidac (Falcons and Caracaras) 
Subfamily: Falconinae (Falcons) 
Genus: Falco (Falcons) 
Subgenus: Cerchneis (Kestrel Falcons) 


Species: sparverius (American Sparrow Falcon) 
Subspecies: phalaena (Desert American Sparrow 

The name of the bird for taxonornic reference is Falco sparverms 
Linnaeus (but for nontaxonomic use, the English name American 
Sparrow Falcon would be used; the taxonomic reference name may 
be added for possible clarification). This name is called a binomial 
name and the system of nomenclature by which it gets this name, .the 
binomial system. In the example given, Falco is the generic name 
(and is always capitalized), sparverius is the species name (and is never 
capitalized), and Linnaeus is the man who so named it (in other words, 
the describer or authority). The bird of the Southwest was described 
separately and carries the subspecies name of phalaena. The authority 
for this name was Lesson, who described the bird originally as Tin- 
mmciilus phalaena. When transferred to the genus Falco, the name 
phalaena continues as the species or subspecies name, as the case may 
be. The name of the authority is then placed in parenthesis to indi- 
cate the change of genus. The complete citation for the Desert Ameri- 
can Sparrow Falcon thus becomes Falco sparverius phalaena (Lesson). 

The Work of Linnaeus. The binomial system of chissification now 
used by taxonomists for birds comes from the work of Karl von 
_Linnc (1707-1778), a Swedish naturalist (known better by his Latin- 
ized name of Linnaeus). While he was chiefly a botanist, the applica- 
tion of his classification system to zoology is pre-eminent. Its suitable- 
ness to animals was rather soon apparent, and the basic system became 
universally recognized and used by zoologists. It has, of course, been 
much modified and amended since the time of Linnaeus. Over the 
years, animal taxonomists have come to agree upon the Tenth Kdition 
of Sy sterna Naturae by Linnaeus (1758) as the starting point for 
classification and nomenclature. This book listed 4,236 animals of all 
kinds, among which were 564 species of birds, about 15 per cent of the 
number known today. 

Rules of Nomenclature. It is obvious that in the development of 
any body of knowledge, certain procedures should develop and be- 
come traditional. In the case of taxonomy, these have become estab- 
lished as working codes by several recognized organizations. Among 
these are the American Ornithologists' Union Code and the Interna- 
tional Rules of Zoological Nomenclature. The latter code, adopted by 
the Fourth International Zoological Congress at Berlin in 1901, sets 
forth among others the following dictums: 

Zoological names are independent of botanical names. 
Species names are binomial (subspecies names are trinomial). 


Scientific names are Latin or Latinized words. 

The original spelling is preserved unless a typographical error has occurred. 

The name of the authority (the first person to publish a name with a 
recognizable description) follows the scientific name (enclosed in paren- 
theses if the species is later transferred to another genus). 

The first name published in accord with the rules (including a recognizable 
description) has priority over others. 

Linnaeus' Systejna Natnrae y Tenth Edition (1758), is the starting point for 
scientific names. 

The International Commission of Zoological Nomenclature acts as 
a sort of "supreme court." Questions arising under the rules and ap- 
pearing to need formal determination are transmitted to the commis- 
sion through a laborious procedure provided by the rules. Zoologists 
usually accept opinions adopted by the Commission as final. 

Check Lists. Several check lists and species catalogs of birds have 
appeared from time to time giving names accepted by the author or 
body preparing the check list or catalog/ A catalog differs from a 
check list by giving descriptions and often more detailed citations of 
other names that have been used for the same species.) In North Amer- 
ica a committee of the American Ornithologists' Union has from time 
to time prepared check lists and supplements which give the scientific 
names of birds agreed upon as acceptable by the committee on behalf 
of the society. The check list also gives a summary of ranges, both 
breeding and winter. In general, American bird students follow most 
of the check list scientific names, though the designated common 
names have not been so widely accepted. 

The most widely accepted, authoritative world-wide check list 
is the Check-List of Birds of the World (Peters, 1934). It gives 
ranges in general terms and, like most check lists of its type, gives cita- 
tions to the original description. The monumental work, Birds of the 
Americas (Cory and Hellmayr, 1918 ), has special usefulness, though 
it is a catalog rather than a check list. The British Museum Catalogue 
(Sharpe, 1874 1898) and Hand-List (Sharpc, 1909- 1912) are also of 
value. Because most ornithologists are not classical scholars, works 
like Cones' Check-List (1882) or Key (1927) are especially useful 
in giving origins and meanings of scientific names of American birds. 

Taxonomic Practice. Naming a bird necessitates that the new 
namejbc accompanied by as complete a description n,s possibly with 
considerably more detail than that found in descriptions of the late 
eighteenth and nineteenth centuries. This is especially true now that 
only minor separations (and hence difficult to detect) remain. It has 
been estimated that probably fewer than a hundred bird species remain 
unnamed. The usual practice is to name a type specimen of type 


group, a type locality, and perhaps also a type of the other sex. The 
describer usually lists the various races and specimens with which he 
has compared his novelty and presents a statement of these compari- 
sons. This may include a statistical treatment and almost always a 
detailed summary of the ways by which the new form differs from 
those already named. The range and location where specimens have 
been found is also given. Publication generally means appearance of 
the new name and description in a "recognized" journal. The publi- 
cation date usually means the actual date of circulation rather tfran 
whatever calendar date may be imprinted on the periodical. (Some 
periodicals, especially European ones, may appear months and occa- 
sionally years after the imprinted date.) 

Subspecies (often called races or fonm) are rather subjective 
groups. The various subspecies grade (also called inter grade) one into 
another because they are all one species. The validity of subspecies 
names therefore becomes rather substantially a matter of opinion be- 
cause the fineness of the distinctions used varies with taxonomists. 
While some hold that any constant, recognizable difference is enough 
to segregate a race, others hold that the difference must be such that 
a fixed percentage of the race in question can be definitely separated 
from other races. A common percentage suggested is 75 per cent. 

It should be noted that groups additional to subspecies are likewise 
determined subjectively in varying degrees. The larger groupings 
(order, family, genus, and their subdivisions) are not found as identi- 
ties in nature, although they are recognizable in practice. The fact 
that taxonomists freely push around whole groups, combining some, 
elevating some, and subordinating others, illustrates the very real diffi- 
culties in dealing with manifestations not as yet measurable and defin- 
able, and therefore matters concerned largely with subjective deter- 
mination (e.g., c.f. Mayr and Amadon, 1951; and Wetmore, 1951). 

The Number of Species of Birds. In 1758 Linnaeus listed 564 
species of birds known to him. Ornithologists recognize about 8,600 
today; one count gives 8,590 (Mayr and Amadon, 1951). Nearly 
60 per cent of these belong to one order, Passeriformes. I low many 
races there are is a matter of conjecture, but the number "recognized" 
approaches 30,000. The number will grow or shrink with the fineness 
of discrimination or opinion. The number of genera of birds also is a 
matter of conjecture or opinion but probably is between 1,800 and 
2,600. In comparison to the bird world, there are about 3,500 known 
mammals, 5,500 known reptiles and amphibians, and 18,000 known 
fishes. Animal species of the Phylum Chordata total about 36,000. 
In contrast to this, the total known species of insects has been sug- 
gested as perhaps a million. 


Orders and Families of Birds. It was envisioned that with the 
adoption of the binomial system of nomenclature and use of the 1758 
edition of Sy sterna Naturae as a beginning, a stability of names and 
classification would be achieved. This commendable ideal has not 
been realized and seems unlikely to be reached in the coming years. 
Part of this instability arises from the constantly increasing knowledge 
of birds that makes revision in classification necessary and partly from 
the inherent weakness in any system that depends upon "priority" 
(and opinion also) for stability. 

Two subclasses comprise the Class Aves (Appendix II). Archac- 
ornithes consists of the ancient, reptile-like birds, as exemplified by 
Archaeopteryx. The other subclass, Ncornithcs, comprises the "true 
birds." Many of the latter arc as extinct as Arcbaeopteryx\ in fact, it 
would be most unusual if more birds had not become extinct in the 
past 150,000,000 years than the 8,600 living today. Because of the 
ancientness of their times, but one order and one family of Archae- 
ornithes is known. In contrast, the true birds are divided into 31 
orders, of which Passeriformes (the perching birds) is the largest. It 
has some seventy families. In a way, birds parallel mammals, where 
among the mammals the order Rodentia far outnumbers others, just 
as Passeriformes docs among the birds. A list of orders and families of 
birds of the world is given in Appendix II. 


CALMAN, W. T., The Classification of Animals. London: Alcthucn & Co., 1949. 

CORY, CHARLES B., and CHARLES E. HELLMAYR, Catalogue of Birds of the Americas. 
Chicago: Field Museum of Natural History, 1918. 

COUES, ELLIOTT, Key to North American Minis. Boston: Dana Estcs & Co., 1927. 

DOBZHANSKY, T. G., Genetics and the Origin of Species. New York: Columbia Uni- 
versity Press, 1941. 

GRASSK," PIERRE-P., Traite de Zoologie, Oiscaux, Vol. 15. Paris: Atasson et Cie., 1950. 
*LACK, DAVID, Darwin's Finches. New York: Cambridge University Press, 1947. 

At AYR, ERNST, Systematics and the Origin of Species. New York: Columbia University 
Press, 1942. 

M\\ R, ERNST, and DKAN AMADON, A Classification of Recent Birds. American Museum 
Novitates, No. 1496, 1951. 

MAYR, ERNST, E. GORTON LINSLEY, and ROBERT L. USIW.ER, Methods and Principles 
of Systematic Zoology. New York: AlcGraw-Hill Book Co., Inc., 1953. 

PETERS, JAMES L., Check-List of Birds of the World. Cambridge, Mass.: Harvard 
University Press, 1931. 

SHARPE, R. BOWDLER, Catalogue of Birds in the British Museum. London: British 
Aluseum, 1874-1898. 

SHARPE, R. BOWDLER, A Hand-List of the Genera and Species of Birds. London: 
British Museum, 1899-1909. 

WETMORE, ALEXANDER, A Revised Classification for the Birds of the World. Smith- 
sonian Aliscellaneous Collections, 117 (1951): No. 4. 


Bird Adaptations 


For biological purposes, an adaptation may be defined as a variation 
that helps the animal iu surviving and succeeding in its environment 
(Chapter 9). But caution seems definitely needed in interpreting 
adjustment to environment: if animals were always in complete ad- 
justment with their environments, it would hardly seem that any could 
have become extinct. That animals did become extinct seems accept- 
able evidence that adjustments were not always successful. Yet 
while adaptations are usually considered as related directly to survival, 
many variations of plumage and some others are associated not with 
everyday life, for example, but with courtship characters and per- 
formances (Chapter 18) . Tt seems obvious enough that our knowledge 
of birds, great though it may be, leaves much to be learned about 
variations and their role in the life of the bird. In this connection, it 
might well be noted that some characters and variations have been 
termed nonadaptive as distinguished from adaptive ones. Whether 
they are or are not can be decided for sure only when bird students 
have explored all possibilities. As more bird students find out more 
about birds, belief in the nonadaptive nature of variations tends to 
disappear. Perhaps it is the incomplete knowledge of the role played 
by a particular variation that gives the appearance of nonadaptiveness. 
Every character and every variation clearly seems to offer a subject 
of research in bird study. 

Origin of Adaptations. The hypothesis has been advanced that 
structure precedes junction, which holds that among all the variations 
arising in bird racial history, only those survive that can serve the bird 
or to which the bird can become adjusted (Alurphy, 1936). The 



principle of limits of tolerance (Kendeigh, 1934) seems to be essen- 
tially the same. 

All species of Tetraonidae (with the single exception of the Att- 
water Prairie Chicken which is but a subspecies) develop horny 
"snowshoes" on the feet with the approach of winter (Fig. 3-1). 
These help increase the bearing surface of the foot in walking over 
snow. Because the family is boreal in distribution and terrestrial in 
habit, we can logically assume that the bird walked in the snow before 
the snowshoe developed. Hence, the feet carried on the function of 
supporting the body in the snow before the improved structure de- 

Fig. 3 I . With the approach of whiter, all Grouse (except the Attwater 
Prairie Chicken) groin a set of "svowshoes" on the toes. In some Grouse 
living in regions of little snow, the siwwshoes may be very small. The foot 
illustrated is that of a Ruffed Grouse. 

veloped. But perhaps an extension of the toe scales on the foot of an 
ancestral Grouse made the bearer more efficient than its fellows and in 
time a complete scries of horny plates developed. As a result of this 
development, the Grouse was able to extend its range and survive in 
areas where snow abounded during part of the year. In one way of 
looking at this, therefore, it appears that the structure developed first 
and that the function fell in line later. But this occurred a long time 
ago, and we can but conjecture the reasons and processes behind it. 

Environmental Adaptation. Environmental adaptation means 
somewhat the same thing as evolutionary radiation, a matter to be 
covered particularly in the chapters on evolution, distribution, and 
ecological relations. Animals evolving under similar conditions are 
sometimes called ecological homologues. Ordinarily they belong to 
the same class, but some interclass parallels are evident between birds 
and mammals, such as Penguins and seals or Moas and ungulates. 

The vertebrate competitors of birds (except for other birds) are 
primarily the mammals, and perhaps they have mutually eliminated 


one another from many habitats. The true subterranean world belongs 
to mammals, largely of the order Insectivora (moles) and Rodentia 
(pocket gopher). The semisubterranean world (sometimes called 
fossorial) also belongs to the mammals, although a few birds (e.g., 
Burrowing Owl, Petrel, Bank Swallow, and Kingfisher) have taken a 
small share of it. The surface stratum under grass and other short 
plant growth (sometimes called the subvegetation zone), particularly 
in the open range lands, also belongs largely to rodents. But in the 
brush and forest, the stratum just above the ground surface belongs 
largely to the birds, and they even claim much of the forest floor itself. 
Mammals have not become adapted to flitting or jumping from twig 
to twig, limb to limb, or tree to tree efficiently enough to challenge 
the birds. About the best mammals thus adapted for this environment 
are the tree squirrels among Rodentia and monkeys among Primates. 
But these creatures move clumsily compared to a nimble bird searching 
for insects, fruits, berries, and seeds. 

The only mammalian competitors of the birds in the aerial world are 
the bats, and with but a few exceptions, bats use the flying zone only 
at night when most of the birds are asleep, for few birds have invaded 
the night period to the extent that they could repress the bats. Birds 
dominate the marsh and shore more than do mammals. And with their 
great mobility, once away from land, birds as a class are more success- 
ful than mammals "in using lakes, ponds, streams, seas, and oceans, al- 
though some mammals are well adapted for aquatic life (e.g., whales 
and dolphins)* 

It seems entirely probable that, as a general rule, where a major 
mammal radiation can take place or has already done so, birds have 
some difficulty in competing with mammals. The Moas evolved on 
the grasslands of New Zealand (which mammals could not reach) as 
the avian occupant of the "ungulate niche." Though the Moas, both 
great and small, occupied the ungulate niche, they were evidently a 
heavy, ponderous type of animal. The shortening of the tarsometa- 
tarsus attests to this if we may judge by the mammalian world in 
which it becomes shorter in the "graviportal" type and longer in the 
fleet, cursorial animals. The tarsometatarsus of the Ostrich measures 

49 cm.; the Emu and Cassowary lengths are 39.5 cm. and 32.5 cm., 
though the birds themselves are only about half the size of the Ostrich. 
In the Rhea, a fifth the bulk of the Ostrich, the tarsometatarsus is 32 
cm. The Giant Moa undoubtedly weighed in excess of 500 pounds, 
about twice the weight of the Ostrich, and stood 12 to 18 feet high. 
Yet the tarsometatarsus measured only 45 cm. as compared with the 

50 cm. length of the Elephant Bird (Aepyornis waxhnus) that 
weighed about 1,000 pounds (Amadon, 1947b). 



The Pengjjias of the Antarctic and Subantarctic Zones occupy in 
the bird world a niche rather substantially similar to that occupied by 
the seals in the Northern Hemisphere. Both groups have several spe- 
ciesTJTwide distribution in the oceans, and some have even moved 
into the respective middle and low latitudes. Both occupy a niche 
characterized by pelagic life during much of the year with return to 
land during the breeding season. Both have become nearly completely 
marine, though the Penguins can still walk on land while the seal can- 
not. The body form of both groups is streamlined. Kven the heavily 
muscled tail of the Penguin parallels that of the seal. The Penguins 
arose in Antarctica, perhaps in early Cretaceous times. The ancestral 
forms may have been land birds that became flightless in the remote- 
ness of their insular home from which terrestrial enemies and competi- 
tors were barred by distance (Murphy, 1936). The spread of seals 
into the Southern Hemisphere in recent times (they are found in the 
Falkland Islands) may foretell an invasion of the Antarctic realm 
with the result that the mammal will take over from the birds there. 
But it may be that the Penguin is so well entrenched in its niche that 
any invasion of competing seals has mostly failed. Yet it should be 
noted that the leopard seal preys upon Penguins; a predator, however, 
is hardly likely to oust its prey, and a predator should hardly be con- 
sidered a competitor. 


Forelimb Adaptations. There seems to be agreement among biol- 
ogists that the wing of the bird evolved from a walking limb of a 
terrestrial ancestor. Whether the iimnediate reptilian ancestor had 
bipedal or quadrupedal locomotion is immaterial at this point. In the 
course of becoming a wing instead of a leg, the light wrist bones fused 
into two bones. (Two of the fingers have also disappeared and but 
three remain.) This fusion of parts strengthens the end of the wing, 
the part bearing the great primary feathers, and reduces weight at the 
same time (Fig. 3-2). 



I (thumb) 



Fig. 3*2. Bones of the ^ or climb in birds. The numbers indicate the 
digits or "fingers" 



In most birds, the skin now covers even the ends of the fingers and 
thumb, where claws formerly grew, except in the Ostrich (which re- 
tains claws on all three fingers) and in some other species or indi- 
viduals, especially in the young. Three claws may be seen also on the 
wing of Archaeopteryx. Some young birds (e.g., the young of the 
Green Heron) still use the wing in a quadrupedal manner in climbing 
about in the vegetation surrounding the nest. The young Hoatzin of 
South America does likewise; young of common Passerines may do so 
also. A Parrot, too, may climb about like a four-legged animal, even 
as an adult. 

Fig. 3-3. Four different shapes of wings associated with flight pat- 
terns, (a) High-speed, maneuvering wings of a Swift; (b) broad-winged, 
slotted, over-land type of soaring wings as of a soaring Hawk; (c) long, 
narrow, over-water type of soaring wings of the Albatross; (d) short, 
rapid-acceleration type of Bob-white wing. 

The proportionate length of the wing is usually governed by the 
number of secondaries (or vice versa). In some birds, however, a long 
wing may be the result of longer primaries, as in the Chimney Swift. 
Despite its great wing-spread, for example, an Albatross has only ten 
primaries, the same number as a tiny Hummingbird. But the Albatross 
has forty secondaries to the Hummingbird's six. The primaries at- 
tach to the finger and wrist bones, the secondaries to the forearm. 
Additional feathers forming the alula belong to the thumb (pollex). 
As will be seen in the chapter on flight, numerous variations occur in 
the shape of bird wings, just as numerous flight patterns occur also 
(Fig. 3-3). Varied but characteristic methods of launching flight or 
coming to rest also occur. 



In addition to the evident flight modifications, on the wings are 
found a number of ornamentations, of which most occur on the male, 
a fact that indicates the secondary sexual character of such plumage 
(Fig. 3-4). Many ducks sport strikingly beautiful wing adornments 
(Fig. 3-5). The long, spectacular feathers decorating the Birds-of- 
paradise have a bizarre look. The Waxwing has the wing feathers 
decorated with waxlike tips. 

(a) (b) 

Fig. 3*4. Wing ornaments seldom take the fomi of trailing leathers as 
in (a) the Pennant Nightbawk. More common types are white patches 
asm (b) the Willet, or colored markings like those (c) of the Red-winged 
Blackbird. Associated with color patches may be characteristic display 
habits as in (a) and (b). 

Fig. 3-5. Many ducks have striking wing pattern adornments. 
Eider, (b) Golden-eye, (c) Gadwall, and (d) Mallard. 

Tail Adaptations. The chief function of the tail seems to be to op- 
pose the air and thereby to support the rear of the body (Chapter 15). 
The tail may probably function to some extent as a steering apparatus 
(as does the rudder in airplanes and ships), but maneuverability is 
Associated largely with flight control through the wings and their 
muscles, the power plant of birds. Soaring birds have rather broad 
tails while powerful flyers may have narrower ones (Fig. 3-6). But 
birds of rapid, direct flight tend to have short tails and somewhat 
faster wing beats. Swifts, Hummingbirds, and others of fast flight 
have rather narrow wings and almost no tail. A comparison of the 
general tail length of Passerine birds indicates an increase in flight 
speed with decrease in tail length (Table 3 \ } . 



Fig. 3 6. Soaring birds have rather broad tails, open heavily muscled, 
'while fast-ftyi?tg birds way have narrow tails. 

Table 3-1 
Flight Speed and Tail Length 

Average Flight Speed 
(from Cooke, 1937) 

Length of Tail as a 
Percentage of 
Total Length 



The fact that many long-legged wading birds fly with the legs 
outstretched behind is probably of some significance. Most, if not 
all, are birds with small tails. They are also birds with long necks, so 
that the projecting of legs behind and neck ahead balance each other 
(see Fig. 1*15). It is unlikely that the long legs have any great use 
in steering, although air pressure against them may perhaps help in 
their support. In any event, birds that hop about quickly from perch 
to perch hold their legs in a ready position against the body. 

The number of tail feathers, with few exceptions, varies from four 
to twelve pairs (they always appear in pairs in accordance with the 
bird's bilateral symmetry). The Cassowary and Emu have no tail 
feathers and the domesticated Fan-tailed Pigeon may have twenty 
pairs. The ancient Archaeopteryx had a long tail of twenty vertebrae; 
some authorities report that each vertebra supported a pair of feathers 
and some hold that only twelve did so. Increased flight power has 
been associated with a shortening and fusing of the tail elements until 
now only a few vertebrae remain separate, the rest being represented 
by a peculiar bone, the pygostyle (called also the plowshare bone). 



The feathers are inserted in the fleshy mass of tissue which surrounds 
the pygostyle. (Fig. 1'7). 

Scansorial birds like the Woodpeckers, Creepers, and Woodhevvers 
use the tail as a prop in moving Up tree trunks. The Woodpeckers 
particularly brace themselves with it \vhile drilling holes in trees. 
The absence of barbs in the tail of the Chimney Swift results in spines 
which are remarkably effective in helping to support the bird when 
it clings to a vertical surface. 

Just as the wings occasionally bear ornaments, usually in the male 
only, tails also often bear them. But because the tail plays a less im- 
portant part in flight, ornamentation is more common and often 
bizarre (Fig. 3-7). The Motmot has a tail with central feathers pecu- 
liarly developed into a trailing, mothlike enlargement. The swallow- 
tails of the Barn Swallow (our only Swallow-tailed Swallow), Arctic 
Tern, and Swallow-tailed Kite lend grace to their flight. 

Fig. 3 '7. The tall of the bird often bears bizarre ornamentation, (a) 
Scissor-tailed Flycatcher of Texas, (b) Paradise Why dab of Africa, 
(c) Bine Grouse of British Columbia, in full display, and (d) elongated 
middle tail feathers of the Central American Motmot. (From various 


The Emu-wren has six long, almost barbless tail feathers which 
greatly weaken its flight, ornamental though they may be. The breed- 
ing plumage* of the male Paradise Whydah (Widow Bird), an African 
member of the Ploceidae, includes tail feathers more than five times the 
body length, the bird itself being about the size of a House Sparrow. 
It is said that when these feathers are wet with dew, flight is impos- 
sible. The gorgeous tail of the Lyre Bird results from special modifi- 
cations of the outer pair of feathers, which outline the lyre, and its 
six pairs of inner ones, which resemble lyre strings. The European 
Black Cock (Lyrums) has recurved, lyre-shaped outer tail feathers. 
The Lyre-tailed Honey-guide has similar tail feathers, but the outer 
ones vibrate in the air stream to produce sound. 

The ornamental tail of the Peacock involves the upper tail coverts, 
though the rectrices themselves may supply some support. Males of 
the family Tetraonidae spread the tail in display, as do some other 
gallinaceous birds (Fig. 3--7). Many other birds do so also, including 
some Passerine birds such as the American Redstart and Myrtle 

Adaptations of Head Structure. The adaptations in the head 
structure of birds are generally associated with feeding habits, but 
many superficial characters arc associated with breeding seasons. The 
eyes are situated rather high on either side of the head, which gives the 
advantage of a wide vision forward, sideward, and backward, cov- 
ering about three-quarters of a circle (Fig. 4-21). 

The wide variety of bills in the bird world shows well the evolu- 
tionary possibilities in a single organ (Fig. 3-8). While it is impossi- 
ble for us to designate any bill as a general type, several bills do appear 
to be "general purpose" ones. Among these are the bills of gallinaceous 
birds, and especially the members of Corvidae and Tcteridae. They 
can be used to pick flesh apart, tear chunks out of fruit, seize an insect, 
crack a seed, or pick up a small object. The bills of many birds, on the 
other hand, have become so specialized as to be restricted in their use. 
Thus, the bill of the Crossbill serves admirably for opening pine cones 
so that seeds may be extracted but is less effective for most other types 
of feeding. Woodpeckers can chisel open a tree-borer's home, but 
they have some difficulty picking up small objects. The bills of War- 
blers and other insectivorous birds are rather poor instruments for 
cracking seeds, although they are ideal forceps. 

Among the water birds occur many noteworthy bills peculiarly 
adapted to the food habits of the species. The Pelican bill bears a 
capacious pouch, larger than the gular sac of any of its relatives. 
Most Ducks have a bill admirably suited for straining food particles 
from the water, but Mergansers have saw-toothed bills to hold fish. 




Fig. 3*8. Some types of bird bills. 

The bill of the Black Skimmer has an elongated and razor-edged 
lower mandible, longer than the upper one (which resembles the 
usual type). The bird skims above the water (page 441) with its 
bill projecting below the surface to scoop up or to pick up organisms 
(Fig. 3-9). The bills of predatory birds have sharp hooks at the end 
for tearing flesh of prey into chunks for swallowing. The hooked 
bill helps also to hold prey, though this is chiefly the job of the talons. 

Fig. 3 f 9. The bill of the Black Skimmer has a rigid, elongated lower 
mandible. The short upper mandible opens and closes instead of the 
lower. (Photograph by Ivan Tompkins.) 



Cormorants and some other predatory water birds likewise have 
hooked bills. 

Some adornments appear on the bill (Fig. 3-10), which is in line 
with the principle that any especially visible part of the bird may bear 
ornamentation. The bill of the Puffins is a gorgeous, triangular affair; 
its color becomes intensified during the breeding season. The Rhi- 
noceros Auklet received its name from a hornlike, breeding-season 
projection atop the bill (Fig. 3-10). The bill of the Evening Grosbeak 
changes from yellow to a clear green with the coming of the breeding 



(c) ^ (d) 

Fig. 3-10. The variety of bird /;/'//$ indicates the evolutionary possi- 
bilities in this single structure, (a) Toucan, (b) Finch, (c) Hornbill, (d) 
Rhinoceros Anklet , and (e) Curassow. 

Nearly all birds have skin muscles capable of causing feather erec- 
tion on the head. It may change the outline, even giving the bird a 
very grotesque appearance at times. The great variety of crests, 
plumes, and ornaments is indeed remarkable (Fig. 3-11). Many are 
found in the male bird only; some are present only in the breeding 
season. In some species, both sexes possess these adornments, but those 
of the female are smaller or less spectacular. 

The crest on some birds stands erect at all times. Among common 
birds with permanent crests are the Tufted Titmouse, Cardinal, Stel- 
ler Jay, and Cedar Waxwing. The Secretary Bird derives its name 
from permanently displayed plumes at the back of the head that re- 
minded earlier travelers of a bookkeeper and his pens. Sometimes 
the common name indicates adornment, e.g., "crested," "tufted," or 
"crowned." Some birds (the Ruby-crowned Kinglet, for example) 
have a crest that can be flashed at will (Fig. 7-18). 



Fig. 3-11. A few examples of the great variety of bead plumes, crests, 
and other adornments, (a) Hooded Merganser, (/;) Royal Flycatcher, 
(c) Valley Quail, (d) Domestic Rooster, (e) Horned Oivl, (f) Tufted 
Titmouse, and (g) Mountain Quail 

Fig. 3*12. The Crested Curassoiv, or Faisano Real, of Mexico bears 
both a crested head and an adorned bill. The entire bill, adornment and 
all, grows more intensely yellow 'with the coming of the breeding season. 
(By permission from Mexican Birds, by George Miksch Sutton, p. 166. 
Copyright, 1951, University of Oklahoma Press, Norman.) 


totipalmote m fi^^ lobote 

Fig. 3*13. Some types of feet in birds. 

The common barnyard Chicken wears large excrescences on the 
head in the form of combs and wattles. The male has them much 
larger than the female. The Turkey has a bare head with warty 
growths. In at least one bird, the Bell-bird of South America (a mem- 
ber of the Cotingidae), the head adornment has been reported to be 
inflatable. The colors of the various wattles and similar adornments 
vary from red and yellow to orange and occasionally even to blue. 
The u bulbous nose" and bill of the Faisdno Real are yellow, though 
the crest is black (Fig. 3-12). 

Scavenger birds like the Vultures and Condors usually have bare 
heads, perhaps because plumage on the head tends to become exces- 
sively "soiled" in the Vulturine way of life. 

Foot Adaptations. The shape, structure, and condition of the foot 
is related to life habits, and the foot varies no less than the bill (Fig. 
3*13, Fig. 1-12). Although a five-toed hind foot probably belonged 
to the reptilian progenitors of birds, no birds (not even fossil ones) 
have more than four toes. Two is the smallest number of toes found 
among birds; even as few as three toes are not common. 

Scales cover the feet of birds; probably they have been retained 
since reptilian days. Several different types of scale coverings can 
be recognized, varying from numerous scales in gallinaceous birds, 
Shorebirds, and others to a single horny covering in the American 
Robin and its relatives (Fig. 3-14). 

Toes terminate in claws that vary with the habits of the bird, just 
as the foot itself does. The Chicken and other scratching birds have 
strong, nearly straight claws. Perching birds have slender, sharp- 
pointed ones. The claws of some water birds may be much reduced, 
but not so among most land birds. The claw of the rear toe on some 
walking Passerine birds (the several species of Longspur show this 
well) is especially elongated, probably to give additional bearing sur- 
face upon the ground and to prevent rearward tipping. Toes of perch- 
ing birds are rather unsuited to ground travel, or if modified for 


Fig. 3 - 14. The foot of the bird bears a light protective covering of 
scales, probably as a still-useful inheritance frow reptilian times. 

ground \y_ork (as in many Ictcrine species), their perching facility may 
be reduced. 

Two separate adaptations of the foot for swimming have been 
evolved, the common one being the webbed (palmate) foot, as in the 
Anseriformes. In this order, webbing involves only three toes, the 
rear toe remaining free. In Pelicans and their allies, however, the 
webbing includes all four toes, a condition called totipahiate. The 
second development of a swimming foot is by means of lobes (lobate) 
which grow out from each side of the toes. These lobes fold together 
as the foot draws forward and flatten out against the water during the 
backward, power stroke. In the webbed foot, the toes are drawn to- 
gether in moving forward and spread out for the power stroke. 

Predaceous birds have the toes tipped with strong curved claws, the 
whole forming the characteristic talons. These are especially capable 
as grasping and piercing devices for catching and holding prey. 

Wading birds, including Shorebirds, have long toes that give 
greater control in supporting the towering body and give greater 
bearing surface on the soft mud bottom. The long toes of the Jacana 
are especially noteworthy examples of elongation for supporting the 
body on a soft footing. The Jacana can walk over aquatic vegetation, 
a feat made possible by the long toes which can spread across a large 
leaf or several nearby ones. The Gallinules have some faculty also 
for walking on lily pads and other vegetation, and most Shorebirds 
show similar, though not so efficient, tendencies. 

Most birds carry three toes forward and one backward. The 
cursorial type of foot found in the Ostrich has two toes, both toes 
directed forward. One of these toes is small, which shows an evident 
evolutionary trend toward a single-toed, running foot, a singular 
parallel to the evolution of the horse. The toes of Chimney Swifts are 
especially adapted for grasping vertical surfaces. Woodpeckers, too, 
can hold to vertical surfaces; they have two toes pointing forward 


and two backward, though some species have only three toes in all, 
two forward and one backward. A few birds, like the Osprey and 
Horned Owl, can rotate the outer toe to present two forward and 
two backward or three forward and one backward. The "yokeHtocd" 
foot of the Cuckoos and their relatives has two toes forward and two 
backward fn a condition termed zygodactyly. 

The length of the legs in birds varies more or less according to their 
general life pattern. Marsh, shore, and wading birds have the legs 
lengthened in order to hold theTfody" above the water. As a compen- 
sation, the neck has lengthened so that the. bill will still reach the 
ground. Some exceptions do occur, however, as might be expected. 
The Woodcock, although a Shorebird, has short legs in token of its 
terrestrial, woodland habits. The legs lengthen among running birds 
to increase speed over the ground, and the neck of running birds has 
become correspondingly elongated. The Road-runner of southwest- 
ern North America shows the same relationship of long legs and long 
neck; its Cuckoo relatives, by contrast, have short legs and necks. 

Other adaptations among swimming birds are worthy of remark. 
The plumage is heavy and "wet proof." Its thickness supplies a high 
insulation factor, needed more in the highly conductive water medium 
than in air. No doubt the feather coat also helps to increase buoyancy. 
Allied to the insulation afforded by the feathers is a heavy layer of fat 
immediately under the skin, which provides an additional insulation 
layer, just as it does in sea mammals. The feet of many aquatic groups 
(as in the Penguins, Loons, and Grebes) have shifted to the stern 
where they are more efficient paddles than they would be if they 
were set farther to the front. The Loons and Grebes cannot stand 
well on land, but Penguins can do so in an erect position. This erect 
stance gives to them a comical, human aspect. 


* ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 

*ALLKN, GLOVER M., Birds and Their Attributes. Boston: Marshall Jones Co., 1925. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
EVANS, A. H., Birds. London: Macmillan & Co., Ltd., 1899. 

*FISHER, JAMES, Birds as Animals. London: William Heinemann, Ltd., 1939. 

*FISHER, JAMES, Watching Birds. Harmandsworth, England: Penguin Books, Ltd., 

*HESS, GERTRUDE, The Bird: Its Life and Structure. New York: Greenberg, Pub- 
lisher, 1951. 

*NEWTON, ALFRED, A Dictionary of Birds. London: Adams & Charles Black, 1893-1896. 

*PETTINGILL, OLIN SEWALL, A Laboratory and Field Manual of Ornithology. Min- 
neapolis: Burgess Publishing Co., 1956. 

*PYCRAFT, W. P., A History of Birds. London: Methuen & Co., 1910. 

"THOMSON, J. ARTHUR, The Biology of Birds. New York: The Macmillan Co., 1923. 


Body Structure 
and Operation 

An intimate relationship exists between anatomy, the study of body 
construction, and physiology, the study of body operation. I Icnce, it 
seems profitable in studying the anatomy of the bird as a living organ- 
ism to relate structure to function. Physiology itself is related rather 
intimately to ecology, for the former deals with the internal relations 
of the body and the latter with the external ones. Though ordinarily 
clear enough, it is hard (or even impossible) sometimes to rccogni/e 
the point of transition from physiology to ecology or at times to 
separate one from the other. 


Skeletal Structure. The skeletal structure of the bird consists 
primarily of bone and cartilage, along with the necessary soft tissue 
(largely connective) to fasten the parts to each other and to the rest 
of the body.Cfiirds use cartilage much less in the skeleton than do 
other vertebrates, chiefly because the bird skeleton must be rigid for 
service in flighty Hyaline cartilage, now forming the trachea! rings, 
has become calcified and is sometimes termed calcified cartilage, 
though it is still hyaline. Fibrous and elastic cartilage is used sparingly 
for filling in at the ends of bones or for partial support of other 

Bone is the most important feature of the skeletal system and 
reaches a high specialization with respect to economy of material in 
the bird; it is far more rigid than cartilage. The bone is living tissue 
in the sense that nerves and blood vessels permeate it. The skeleton 













, UL.NA 




-Rl BS 









Ft BUL.A' 

Tl BIO- 




Fig. 4 - \.Skeleton of the Domestic Fowl. (By permission from Text- 
book of Zoology, by George Edwin Potter. Copyright, 1938, The 
C. V. Mosby Co., St. Louis.) 




serves four uses in bird life: (1) providing a supporting framework 
for body parts, (2) supplying soHcTattachments for muscles, (3) 
fufmsKing" leverage for movement, and (4) supplying protection to 
delicate tissues. Its role in avian red blood-cell production is not 
clearly known. 

The skeleton acts as an internal framework or scaffolding, some- 
what as a steel frame reinforces and supports the masonry, brick work, 
and trim of a skyscraper (Fig. 4-1). The wings, for example, carry 
most of the weight of the body when in flight, while the legs carry 
all of it when walking, standing, or perching. Even in water, the 
skeleton maintains the relatively uniform shape of the body, though 
support is transmitted through the body itself rather than through 
the limbs. 

The muscles of the body usually attach to the skeleton through 
flesh or tendons, their fixed end being the origin, the movable one the 
insertion. Thus the contraction of the muscle pulls against the rigid 
skeleton to cause movement at the insertion. Correctly speaking, 
muscles perform by contraction only; hence, additional muscles or 
some retractive forces must be present to reverse the contraction. The 
places of attachment often become flattened, ridged, or otherwise 
much increased in surface area for greater attachment power of the 





Fig. 4 -2. (a) The leg and toes are operated by muscles and tendons. 
Those operating the toes pass behind the tarsal joint and through the 
annular ligament to insert on the respective toes. Roosting causes tension 
on the tendons by bending the "heel and knee" joints so that the toes 
firmly clasp the perch, (b) Bracing of the pectoral girdle and the muscles 
that move the wings. 



muscle. (Variations in shape of the bones under the influence of mus- 
cle attachment become very great indeed, and each group and even 
each species tends to have bone variation/such as ridges, crests* tuberos- 
ities, arid processes so characteristic as to make most larger bones and 
many smaller ones identifiable, sometimes to the species. The most 
elaborate of these bony modifications for muscle attachment appears 
in the thoracic basket of the bird. The sternum has a greatly devel- 
oped keel for the attachment of the enormous flight muscles, so that 
the muscles operating the respective wings pull against each otfier. 

Fig. 4*3. Muscles perform their work by using levers of the first , 
second, or third class as indicated in these three examples. The arrow 
shows the direction of power (P), W indicates the weight, and F the 

The flat ribs contribute to the streamlining and lightness of the body 
by reducing the need for padding over the ribs. The sternum is also 
braced against the wing bones themselves through the rib structure 
and pectoral girdle (Fig. 4-2). The muscles do their work through 
systems of "cables, pulleys, and levers" in accord with physical prin- 
ciples (Fig. 4 3 ) . 

The cranium that protects the brain from injury shows clearly the 
protective function of the skeleton. In the same way, the vertebral 
column protects the spinal cord. The axial skeleton consists of the 
skull, fibs, sternum, and backbone, while the appendicular skeleton 
includes the remaining bony structure. 

([The skull of the bird shows most strikingly the trend in body 
structure among birds for reduction in weight by fusing, thinning, 
and eliminating parts. The many bones comprising the skull can be 
distinguished fairly well in embryos and young birds. But by the time 
adulthood is reached, they have fused so completely that it is difficult 











Fig. 4-4. The various separate bones of the skitll (Domestic Chicken) 
fuse during growth, and most of the sutures and individual bones are not 

M. pseudotemporalis 


hinge \ 



proc. supram. 
^proc. retr. 

jugal bar 


foramen mand. 




M. protractor^ 

M. pseudotemporalL 

M. depressor 

M. adductor 

PROTRACTORS ( Gaping ) ADDUCTORS ( Seizing ) 

Fig. 4 - 5. The junction of the bones and muscles shown in this illus- 
tration of the "kinetic" skull. (From William /. Beccher, "Adaptation for 
Food Gathering in the American Blackbirds;' Auk, 




or impossible to distinguish most of them except by locatiorij (Fig. 
4-4). Many of the old reptilian (and even earlier) parts have been 
retained; few have been lost entirely. The lo^rj^ fprjjxample, 
still articulates with the skull through the independent qTiaffratlTBones. 
The upper mandible usually is rather fixed, but it bends freely afflre 
skull in Parrots. Many young and some old birds, particularly in 
Paridae, have considerable flexibility between the skull and upper 
mandible. The lower jaw of the Black Skimmer is held rigid and the 
upper one opens in feeding (Fig. 3-9). The quadrates themselves 
articulate with the ptery golds, which in turn bear upon the palatines 
and basisphenoids (Figs. 4-4, 4-5). An elaborate but efficient muscle 
system operates the bill (Figs. 4*5, 4-6). 

Fig. 4 6. The muscle pattern of the Cowblrd bill. 

Protractors: Open the bill. (/) M. depressor mandibulae (depresses 
lower mandible)-, (2) M. protractor quadrat! (elevates upper mandible). 

Palatine retractors: Draws upper mandible downward. (3) M. ptery- 
goideus dorsalis: (3a) anterior, (3b) posterior; (4) M. pterygoideus ven- 
tralis: (4a) anterior, (4b) posterior (underlies 4a)\ (5) M. pseudotem- 
poralis profundus. 

Mandibular adductors: Combine to draw lower mandible upward. 
(6) M. pseudotemporalis superficialis; (7) M. adductor mandibulae: (la) 
externus superficialis, (Ib) externus medialis, (7^) externus profundus, 
(Id) posterior. (From William ]. Beecher, "Adaptation for Food Gather- 
ing in the American Blackbirds" Auk, 6S(1951):418.) 

/The brain case of the bird is larger than that of reptiles (because 
tnfe bird has a larger brain) and has moved backward to make room 
for the greatly enlarged eyes. Heavy cartilage, evidently commoner 
in earlier forms, has given way to the stronger bone, and sizable 
cartilage persists only in the septum between the orbits and the eth- 
moid region. (Some separate bones largely of cartilaginous origin 
are the exoccipitals, periotics, alisphenoids, orbitosphenoidSj and eth- 


molds.} The skull articulates with the vertebral column through a 
single occipital condyle, which has shifted to the underside of the 
skull itself^ 

The ancestral gill arches gradually have come to serve new func- 
tions, though numbers five and six are missing in the bird and number 
four partially so. The location of the old gill arches in present-day 
birds follows: 

1. Pterygoid, quadrate, and Meckel's cartilage 
II. Stapes, hyoid apparatus 

III. Hyoid apparatus (Fig. 4-7) 

IV. Absent (some may possibly be used in hyoid) 
V. Absent 

VI. Absent 
VII. Tracheal cartilage 

Fig. 4-7. The third ancestral gill arch has become modified to form 
the hyoid apparatus supporting the tongue. 

The pectoral girdle of the bird braces the bony basket, especially 
the coracoscapular-humeral joint and sternum, against the pull of the 
great flight muscles. The coracoids and clavicles serve as powerful 
braces, while the scapula has become narrowed and attached by soft 
tissue as it lies athwart the ribs. Together the united clavicles are 
called the furciila (also wish-bone, pulley -bone, and merry -thought). 

The pelvic girdle consists of the ilium, ischiwu, and the pubis, to 
which are attached by various degrees of fusion the two sacral or 
pelvic vertebrae to form a rigid structure for body support in stand- 
ing and walking. 

The forelimb bones of the bird consist of the humerus of the upper 
arm, the radius and ulna of the forearm, the carpals of the wrist, 
metacarpals of the hand, and the phalanges of the digits. The humeri, 
radii, and ulnae remain free, but the carpals and metacarpals show 
more or less fusion. Only the first (pollex or thumb), second, and 
third digits remain in the bird. The first supports the alula (which 
forms a wing slot with the main wing), the second supports the pri- 
maries, and the third has largely- degenerated or fused with the second 
(Fig. 4-8). 






Fig. 4-8. Wing bones of a bird. 

The thigh of the hind limb projects from the pelvic girdle, some- 
times almost horizontally. The knee joint bends backward as in man 
(though the knee and thigh usually rest hidden from view in the 
feathers). The cms (drumstick) bends backward at the knee to the 
heel joint, where the leg bends sharply forward to the foot (Fig. 4-9). 




Fig. 4*9. The leg bones of a bird. 

The lower part of the leg usually has no feathers and popularly is 
called the tarsus, though technically torsotnetatarsus would be better. 
The bird "normally has toes one, two, three, and four; the fifth never 
appears except as an early embryological transient. The number of 
phalanges characteristically is two for the first toe, three for the 
second, four for the third, and five for the fourth, one greater than 
the toe numbering. But various modifications in number occur. 

The long bones of the leg are the femur (which articulates with 
the pelvic girdle and at the knee joins the tibiotarsus) and the fused 
and degenerate fibula. The lower end of the tibia is fused with sev- 
eral of the upper tarsal bones. The remaining tarsal bones are fused to 
the metatarsus to form the tarsometatarsus. The ankle joint of the 


bird lies between two sets of tarsal bones fused, respectively, to the 
tibia and metatarsus. This is substantally as in many reptiles but 
markedly different from the structure in mammals. (Properly we 
should not call the bones tibia and metatarsus but should refer to them 
as tibiotarsus and tarsometatarsus, respectively, with the mtratarsal or 
wesotarsal ankle joint lying between them.) 

The angles that the joints of the leg make with each other arc pecu- 
liarly suited for launching the bird into the air by straightening the 
legs even as the running mammal can get off to a fast start by 
straightening the angle of the legs in the initial leap. The tendons 
which flex the toes run behind the leg. When the bird squats down 
on a perch, as at roost, the bending of the leg joints tightens the grasp 
of the toes on the perch. Only when it springs up will the toes open. 
This also accounts for the forward projection of the raptoral foot to 
seize a prey or of the perching foot to seize a perch. 

Muscular System. Although the muscles of the body (the study 
of muscles is termed viyology) serve chiefly to generate motion by 
contractions that produce power (transmitted through tendons and 
bones), they serve also for support and protection. The air pressure 
against the body, tail, and wings supports the air-borne bird, but a 
layer of muscle underlies the skin; the internal weight rides on the 
buoying air through the intervening muscles. Air pressure upon the 
wings and tail gives support transmitted in reverse over the tendons 
and muscles to the bony framework; it is the muscles that hold the tail 
and wings firm against air pressure (Fig. 4-10). The muscles, further- 
more, offer some measure of protection to internal structures, such as 
the nerves and bones buried deep in leg muscles. 

There exist three kinds of muscle, smooth or involuntary, striated 
or voluntary, and cardiac, found, for example, in the viscera, legs, and 
heart respectively. Aluscles like those of the wings or legs are cov- 
ered by a connective tissue sheath (fascia) continuous with the ten- 
dons which, in turn, fuse with the periosteum of the bone. Several 
muscle fibers may be ensheathed to form bundles of muscles. In the 
case of the wings, long tendons make it possible for the heavy power 
muscles to be located at the center of the body (and below the 
wings), which relieves the wings of much immediate, cumbersome 
weight. Tendons may become calcified, as may be seen in the legs of 
many mature birds like the drumstick of a Thanksgiving Turkey. 
As would be expected, the muscle system of the bird shows spectacu- 
lar adjustments to the needs of flight. The oblique muscles of the 
abdominal region, well developed in mammals, are reduced. The 
same may be said for dorsal muscles, reduced in birds in favor of a 
rigid thorax. The neck, however, has great flexibleness. 




irff.(Pcaud. fern.) 

F.p et p d 

Tib cart. 



Fig. 4-10. Superficial wing and leg muscles of a Crow, (a) Dorsal view 
of right wing, (b) View of left leg. (a) From George E. Hudson and 
Patricia J. Lanzillotti, "Gross Anatomy of the Whig Muscles in the 
Family Corvidae," American Midland-Naturalist, 53(l953):l-44. (b) 
From George E. Hudson, "Studies on the Muscles of the Pelvic Appen- 
dage in Birds^ American Midland-Naturalist, 18(1931):100-108). 





Digestive Systems. The digestive system of the bird includes the 
mouth with its horny beak, pharynx, esophagus, stomach, small in- 
testines, and cloaca (Fig. 1-7). The crop is part of the esophagus, 
and the gizzard is part of the stomach. Food may pass through the 
digestive system in four hours and sometimes less time. The tongue 
serves chiefly to move the food within the mouth after it has been 
picked up by the bill and passed into the mouth, usually with the aid 
of a quick, forward jerk. The hyoid apparatus, consisting of the 
by old and branchial arches (Fig. 4*7), supports the tongue. The 
tongue is variously modified in many birds according to their food 
habits (Fig. 4-11). In the Woodpeckers, for example, the prongs of 
the hyoid apparatus pass back along the under side of the skull and on 
around to the top, even as far forward as the base of the bill (Fig. 
4-12). The tongue itself is barbed and spear-shaped (Fig. 4-11). 

Fig. 4* 1 1. The tongues of birds vary according to food habits, (a) 
Brushing tongue of Williamson Saf>sucker, (b) nectar-extracting tongue 
of Anna Hummingbird, (c) generalized tongue of American Robin, and 
(d) larva-spearing tongue of Red-shafted Flicker. (After Leon L. Gard- 
ner, "The Adaptive Modifications and the Taxononric Value of the 
Tongue in Birds,' 19 Proceedings of the United States National Museum, 
61(1925), No. 2591.) 

Fig. 4-12. The tongue of the Woodpecker may be run out beyond the 
bill because of the long branchial arches. (After Elliott Coues, Key to 
North American Birds, p. 174. Boston: Dana Estes Co., 1903.) 


Table 4-1 lists several functions of various tongues along with ex- 

Table 4- 1 
Examples of Functions of Some Bird Tongues 

Function Example 

Probe and spear Woodpecker, Nuthatch 

Sieve Duck 

Suction tube Sunbird, Hummingbird 

Brush Sapsucker 

Rasp Vulture, Hawk, Owl 

Fish holder Pelican 

Finger Parrot, Finch, Crossbill 

Salivary glands are present in some birds, though they are less in- 
volved in digestion in birds than in mammals. The Flicker uses its 
salivary glands to produce a sticky fluid with which to entrap ants 
on the tongue; other Woodpeckers do so to a lesser extent. The 
famed "bird's-nest soup" of the Orient is made from the gelatinous 
product of the salivary glands that Swifts (C oil oc alia) use for holding 
the nests together. Other Swifts use secretions of the salivary glands 
in cementing their nests together, as do Swallows also. 

From the pharynx, the trachea leads off to the lungs and the esoph- 
agus to the stomach. The glottis in the floor of the pharynx con- 
trols the passage of air or food into the proper channel. The esophagus 
is a collapsible tube equipped with circular muscles which by peristalsis 
propel the food to the stomach and sometimes regurgitate it. The 
lower portion of the esophagus in many birds has become enlarged 
into a storage chamber called the crop (sometimes craw), which 
appears chiefly in grain feeders like the Chicken. In some birds, crop 
glands soften the food (and possibly digest it slightly), but in most 
birds the crop only stores food before passing it on for digestion. The 
Hoatzin has a strongly muscular crop for squeezing the juices from 
succulent leaves. The Pigeon has long been noted for its production 
of "pigeon milk" by breakdown of epithelial cells in the lining of the 
esophagus. This milk is chemically rather similar to mammalian milk. 

The stomach proper consists of a forward portion merging with 
the lower esophagus (called the proventriculus) into which the di- 
gestive glands empty their contents for mixing with the food before 
it is passed on to the gizzard. The gizzard, the second part of the 
stomach, is a heavy-walled, muscular organ in seed-eaters and other 
birds that feed upon hard foods. It contracts powerfully two or three 
times a minute during digestion. The thick-walled p^irri r fliirlH in 


_ the food by gravel or grit taken in through the mouth and 
retained in the gizzard for this purpose. In areas where hard seeds are 
available, they may substitute for some of the grit; both grit and seeds 
may become worn down in time by continued use. The size of the 
grit varies greatly with species, from practically sand in Passerine 
birds to cobblestones weighing as much as 5 1 / 2 pounds in the extinct 
Moas of New Zealand. The grit usually follows rather closely the 
size of food particles, though averaging larger than the average for 
food items. Birds pick up grit where it is available; Penguins, for 
example, gather their gizzard stones from the sea bottom. Some birds, 
such as Geese, may have especially heavy grinding discs, which may 
be aided by grit. Yet grit does not seem to be an essential item of life, 
for birds can live without it. But digestion in birds customarily using 
it seems more efficient with grit than without it. 

The muscular condition of the gizzard varies with the food habits 
of the species. In meat and flesh eaters, the gizzard is soft compared 
to that of a seed cater; animal foods are easier to break down for 
absorption. The muscularity varies also with the food habits of the 
individual. If a bird feeds upon soft foods over a long period, the walls 
of the stomach will become soft and flabby. An increase in the 
amount of hard foods will result in increased gizzard strength. The 
gizzard lining in most birds is shed and renewed continuously like the 
skin of man. But in the Cuckoo, it is reported to be shed all at once 
and cast off through the mouth. The male Hornbill wraps food in an 
envelope secreted by the gizzard lining for presentation to the im- 
prisoned incubating female. 

Comparative studies of the crop and gizzard contents indicate that 
a delay of about an hour ensues between the peak load of the crop 
and that of the gizzard, which may indicate time taken in softening 
and mixing foods in the crop and proventriculus, as well as the actual 
time in passage. (Food may remain in the crop for two to twelve or 
more hours.) The outlet from the gizzard is at the top, so that gravity 
tends to hold heavy particles inside. The pyloric imiscles at the 
outlet restrict passage of hard and unground foods. Heavy and hard 
objects, like lead shot, for example, remain in the Duck stomach. 
Lighter objects and even grit may be allowed through and into the 
duodenum, the forward part of the small intestine. Some control ap- 
parently can be maintained over the grit, for it may pass out of the 
gizzard when abundant and be retained by some birds for as much as 
a year when it is scarce. "Hunger contractions" of the crop, pro- 
ventriculus, and gizzard may begin before all food is gone, relatively 
earlier in birds than in mammals. This may be associated with the 
higher metabolism and "high-speed life" of birds. 


The small intestines may be rather large and long in vegetation 
feeders, but they tend to be shortened in most birds. Their chief 
function is the further breakdown of food and the absorption of 
nutritive substances; in this they are particularly efficient. The small 
and large intestines are indistinguishable in birds, but paired caeciiws 
(useful in cellulose digestion though apparently not essential to life) 
are attached at the junction of the two. The intestinal tract ends in 
a cloaca, into which the urogemtal sy stein also empties. Opening off 
the cloaca in many birds is a structure known as the bursa of Fabricius. 
The intestinal tract (including the cloaca) shortens in accordance 
wmt-the avian practice of reducing weight where possible. It is 
shortened most in the more migratory species and in those feeding 
upon the more concentrated types of foods. TSome representative 
lengths of intestines in inches, as compiled from various sources, are 
given in Table 4-2. 

Table 4-2 
Comparative Length of Bird and Intestines 

Length Length of 
Species of Bird Intestines 
(in.) (in.) 

Length Length of 
Species of Bird Intestines 
(in.) (in.) 

Barn Swallow 



Prairie Chicken 

. 29 

55 feet 

House Sparrow . ... 



Gannet , 

Burrowing Owl 


Long-eared Owl 

Herring 1 Gull 

Gambel Quail . . . 

Jungle Fowl 


White-fronted Goose. . 
Sandhill Crane . . 

Rock Dove 

Horned Owl . ... 


Black-bellied Tree Duck 
Black Vulture 

\Vhooping Crane . ... 

Canada Goose 


Trumpeter Swan 

Wood Duck 


Accessory structures lying against the intestinal tract are the 
pancreas, gall bladder, and liver. The liver varies in size from about 
1.25 per cent to 2.0 per cent of the body weight in various birds 
(Table 4-3). In the small intestines, bile from the liver alkalizes the 
food mass and also supplies bile salts used in fat digestion. The pan- 
creas supplies enzymes which convert starches to sugars and others 
which act further in protein digestion. Other enzymes are added by 
the glands of Lieberkiihn. The chief digestive enzymes and their 
activities are tabulated in Table 4-4. 



J i 

^ o 




. S 


S B tr 

ro rH 

tx 00 
Ol Ix 







tO rH 

OJ 01 

VO tx 

O 10 

00 to 


rH^ QQ 

vc VO 

to tx 

IO cr; 

ON r 




Tf rH 

vo ON 

to ON 

01 rH 

vo tx 












O* r*) 

00 rH 





vo 01 

O rH 

O O 

t CO 




d d 

O O 

O O 

o o 





rH CM 


O r- 
rH 0] 

tx ON 

01 00 

rH tx 
VO tx 

rH OJ 

tx 00 

O 00 

ON 00 
O 00 

vo ro 
<*5 iO 




d oi 

01 rH 


ro CO 

tX (V] 

-t 01 

rH rH 

ON rH 

ON rH 

rH rH 

rH rH 

O rH 

O rH 

rH 01 





r ~ l 

r 1 


O vo 

n' d 




co d 

rH rvj 

rH O 

d oi 

rH ON 
rH ON 

00 tx 

*t 9 

CO 01 

q co 


10 IO 
PO iO 

d -H 






VO 00 
01 o 

ON rH 

CO fO 

ro ON 


O 10 
tx r>| 

IO rH 

ON vo 

co ON 

01 r/3 

Ix 01 

t tx 

00 T 

VO *5 



01 NO 

01 if 



*i- o 

0] O 

rH Q 

IX rH 

co Ol 

oi o 

CO rH 

O rH 

rH rH 


O 01 

rH OJ 


O tx 

01 01 


10 01 
rH <*) 

r- n 

f*; to 

ON 01 

<^l to 
iO <N 


ON -T 

rH tx 
10 01 

ON -t 


(\ * 1 " 

O tx 




f Tf 

CO 01 

co *e 

d -r 

O f 

01 <N 

rg oi 


rH rO 

Ol PC 

rH irj 

rH Tf 

01 CO 







01 -r 




01 q 


CO 1; 

co q 


00 to 

t l> 






- 1 



O O 

01 o 

ON co 


01 O 

CO rH 
O O 


re ro 
O O 

tx to 

0] o 

O O 

01 0] 



O O 

rH 01 


O O 


O O 

O O 



O O 

O O 







00 O 

rH O 


VO 00 
10 O 


rH O 

O O 

vo to 


o o 

o o 

O O 









o o 

o o 

O O 

o o 

O O 


d o 

O O 


o o 


00 rH 

to m 
ON 10 

ri ON 

rH IO 

ON tx 

<N 00 

re CO 

to 01 

00 *t 

tx 01 

tx CO 

co 01 


d 01 

OJ fr) 

0] vo 
ON to 

g 4 

oj o 

vd d 



t o 

ON - 1 

OJ 0| 

01 O 

CO rH 

rn CO 

r-i **5 

rH t 

rH -f 

01 co 





























































t . 


mission f 



White Pelican .... 





Turkey Vulture . . . . 

Sparrow Falcon 


Ring-billed Gull . . . 



House Sparrow . . . 

House Sparrow . . . 

Bronzed Crackle . . 

v o 




Table 4-4 
Chief Digestive Enzymes and Activity 

Chief Enzymes Source Activity 

Pepsin Gastric fluids Protein to proteoses and peptones 

(Acids) Brunner's gland Acidifies 

( Alkalines) Liver Alkalizes 

(Bile salts) Liver Fat digestion 

Amylopsin Pancreas Starch to maltose 

Trypsin Pancreas Protein break down to peptids 

Steapsin Pancreas Fat to glycerol, fatty acids 

Enterokinase Liebcrkiihn's gland Activates trypsin 

Maltase, Lactasc, Invertase. . . Lieberkiihn's gland Starch breakdown 

Erepsin Lieberkiihn's gland Protein breakdown (peptids to 

amino acids) 

Respiratory System. The respiratory system of birds bears the 
expected resemblances to that of other vertebrates, but in many 
respects it is surely the most remarkable of all, for there are many 
modifications in structure to meet the peculiar needs of a flying or- 
ganism. It probably has the physiologically most efficient of lungs. 
The size of the lung varies from 0.20 per cent of the body weight in 
weak flyers to six or more times that percentage in stronger flyers. 
Air passes in through the nostrils or the open mouth and enters the 
trachea through the glottis, a control valve. The united eustachian 
tubes, which equalize air pressures in the ear, open into the pharynx. 
Air passes down the trachea or windpipe, a tube supported by cartilag- 
inous rings. The trachea divides at the lower end into two bronchi, 
each going to a lung. At or near the junction of the bronchis is the 

Attached to the respiratory system is a series of air sacs that pene- 
trate the various parts of the body. The inter clavicular air sac in the 
Domestic Chicken lies in the angle of the ]urcu\um (wishbone) and 
against the crop. Lying above this are the cervical air sacs. The 
pectoral or pretboracic air sac will be found between the two halves of 
the pectoral muscles of the breast. This air sac extends into the hollow 
humeral bone of the wing and connects also to the interclavicular air 
sac. Another large air sac penetrates the oblique septum and a smaller 
one forward of it envelopes the heart. Lying along the viscera will be 
found the large visceral or abdominal air sacs. Though probably best 
known for the Domestic Chicken, the exact operation of the air sac 
system is not clear. It functions to carry heat out of the interior of 
the body and from within the large muscles; it also indirectly helps 
oxygenate the blood by reinforcing the lung system. All interchange 
of oxygen and carbon dioxide, however, takes place in the lungs 
proper, little or none in the air sacs. 



Two primary bronchi lead from the trachea to the respective lungs 
where they widen out as the vestibulum (with four ventro-bronchi 
arising from each) and the we so bronchi (from which arise seven 
dorsobronchi ) . The abdominal air sac arises from the terminal end 
of the mesobronchus', the post-thoracic air sac is attached to the lateral 
bronchi, which themselves arise from the lateral surface of the meso- 
bronchus. The abdominal and post-thoracic air sacs are termed 


Fig. 4-13. Diagram of the right lung of a bird, ventral aspect. The 
outline of the lung is indicated by a dotted line. Of the anterior air sacs, 
only the prethoractc sac is shown. Of the later obronchi, the only one 
completely shown is the one which forms the post-thoracic sac; only the 
beginning of the laterobronchi is indicated. The recurrent bronchi are 
not reproduced. Only the parabronchi arising from one ventrobronchus 
branch and one dorsobronchus branch are shown as a series of connected, 
parallel tubes. Actually, the parabronchi arising from a ventrobronchus 
anastomose with the parabronchi not only of one but of many dorso- 
bronchi and vice versa, (h) primary bronchus, (ve) vestibulum, (m) 
mesobronchus, (v) ventrobronchi, (p) parabronchi, (d) dorsobronchi, 
(1) laterobronchi, (pr) prethoracic sac, (po) post-thoracic sac, (ab) 
abdominal sac, (le) "guiding dam" (After E. H. Hazelhoff, "Structure 
and Function of the Lung of Birds," Poultry Science, 30(1951)3-10.) 



posterior air sacs. Anterior air sacs (prethoracic, inter clavicular, and 
cervical) arise from the ventrobronchi (Fig. 4-13). Parabronchi, aris- 
ing from the ventrobronchi and dorsobronchi, merge (anastomose) 
so that air may pass through the dorsobronchi, parabronchi, and 
ventrobronchi (d-p-v system) as well as from the mesobronchus in 
the vestibulum (Hazelhoff, 1951). 

Much of the knowledge about bird respiration is conjectural and 
subject to differences of opinion among anatomists. But in the Do- 
mestic Chicken, it is believed that most and perhaps all of the air 
circulates through the d-p-v system in the same direction of passage 
during both inspiration and expiration. This seeming anomaly is 
believed to result from constrictions acting as diversion structures at 
the junction of the mesobronchi and dorsobronchi during expiration 
and at the junction of the primary bronchi and ventrobronchi during 
inspiration (Fig. 4'14). Aerodynamic conditions alone control the 

Fig. 4-14. Greatly shtiplified model of a bird's lung. (A) expiration; 
(E) inspiration. In both phases the air makes the complete circuit (ve-m- 
d-p-v). Whether the weak current from ve to m actually occurs in a 
bird's lung is uncertain. (After E. H. Hazelhoff, "Structure and Function 
of the Limg of Birds," Poultry Science, 30( 1951):3-10.) 

circulation of air; no valves or sphincters are present. The air in the 
posterior air sacs passes through the vestibulum-mesobronchus system 
and some additionally through the d-p-v system. The CO 2 content 
of the posterior air sacs in the Domestic Chicken averages 2.6 per cent, 
that of the anterior sacs 5.4 per cent. The latter receive all of their 
air rather than only a part from the d-p-v system; this accounts for 
the difference in carbon dioxide content (Hazelhoff, 1951). 

The strong thoracic basket of the bird formed by ribs, sternum, 
and backbone has sufficient elasticity to expand and contract by 
numerous respiratory muscles so as to cause inhalation and exhalation 



of air. These muscles are controlled by the section of the spinal cord 
immediately below the medulla oblongata. The hydrogen ion con- 
centration in the respiratory center at the top of the spinal cord and 
the rise of carbon dioxide pressure in the blood seem to stimulate the 
breathing rate. 

Circulatory System. The circulatory system in the bird consists 
primarily of the lymph apparatus, and the heart, arteries, veins, and 
blood (Fig. 4-15). The circulatory system follows the higher reptilian 
pattern, but the postcaval vein connects directly to the renal portal 
system. The heart is large, especially in the more powerful flyers. It 
may vary from 0.40 to more than 1.50 per cent of the body weight 
(Table 4- 3). 


subclavian artery, 

F^carotid artery 

vena cava 




Fig. 4*15. The bird circulatory system. (Left) Arterio-venous circu- 
lation to the liver and kidney; (right) the right aorta only remains and 
from it rise lesser arteries. 

Blood enters the right auricle from the systemic venous system 
and passes into the right ventricle, from which it proceeds into the 
pulmonary arteries and then to the lungs. Following oxygenation, the 
blood passes back to the lungs through the pulmonary veins to the 
left auricle, from which it proceeds to the left ventricle and out 
through the right aorta. There is no complete aortic arch in birds; 
the right half of the fourth arch persists and the left customarily dis- 
appears (Fig. I'll). 

The blood which passes out through the arterial system goes 
through it to all parts of the body. The arteries diminish to arterioles 


and finally arterial capillaries, which merge with the venous capillaries. 
These increase in size to become venules and finally veins for return- 
ing blood to the heart. The part of the venous system operative in 
the viscera, particularly in returning the blood through the great 
liver, is the hepatic portal system. That gathering blood from the 
lower limbs is the renal portal system, and the whole is called the 
systemic venous system. 

The Blood. Blood arises from the mesoderm and consists of the 
plasma, a slightly alkaline (pH 7.5) fluid, and the corpuscles carried 
in it. It is reported to form about 6.5 to 8 per cent of the body weight. 
Two kinds of corpuscles are found in birds, red (erythrocytes) and 
'white (leucocytes). Blood contains salts and other compounds, so 
that it is a complex substance of slightly higher specific gravity than 

The enucleated red cells carry oxygen by means of hemoglobin 
(plasma may dissolve some oxygen also). From 2,000,000 to 7,645,000 
erythrocytes per cubic millimeter have been reported for birds; large 
birds have fewer per cubic millimeter than small birds. The average 
erythrocyte measures about 12 microns long by 6 microns wide, which 
is substantially larger than in man. The red cells of the Osprey are 
the largest reported in North American birds (16 microns) and those 
of the Carolina Chickadee (6.0 microns) the smallest (Bartsch et al, 
1937). The leucocytes are larger than the erythrocytes and are pres- 
ent in smaller and more variable numbers. They serve chiefly to 
combat disease organisms (in the course of which they increase 
greatly in numbers) and to assist general body repair operations. In 
addition to the corpuscles, certain other bodies in the blood known as 
thrombocytes act during a case of injury to form a blood clot over a 
wound. The blood of a bird usually clots within 4 to 5 minutes. 

The plasma is a very complex fluid consisting of 80 per cent water 
and containing at various times dissolved food particles, 'waste prod- 
ucts, hormones, enzymes, cnitibodies, fibrinogen, and other compounds 
needed in the body. All these things carried in the blood stream are 
taken up from and given off to the body tissues by osmosis. Hence, 
all substances for transmission to the tissues must be in solution. 

The fluid of the open lymph system is lymph, closely allied to 
blood plasma, which contains white corpuscles. There are lymph 
nodes variously placed throughout the body of most birds (absent 
in Galliformes and Columbiformes, but some are found in most water 
birds). Lymphoid structures in the bone marrow of some birds may 
compensate for lack of nodules. Lymph collects in lymph vessels for 
return to the blood-vascular system. 







kidney fc$j 


ureter and 
vas deferens 





Fig. 4 16. The iirogenltal system of the bird (Domestic Chicken) in 
ventral view. 

Excretory System. Although the unused parts of food in the 
digestive tract are eliminated directly through the cloaca, the term 
excretory system customarily applies to the urinary apparatus (Fig. 
4-16). It is involved intimately with the genital system and directly 
in the excretion of the wastes of metabolism. We must not forget, 
however, that a large measure of metabolic waste is eliminated through 
the respiratory system as carbon dioxide and perhaps other gases. 
Furthermore, waste products are deposited in the developing feather 
and pass to the outer world at the next molt. 

The avian kidney (metanephros) is packed into the caselike under 
side of the pelvis. Three parts can be identified, the anterior, middle, 
and posterior lobes. The blood carrying metabolic wastes passes from 
the renal artery and renal portal system into the kidney and into a 
capillary loop, the glomerulus, surrounded by a cup called Bowman's 
capsule. The whole is called the renal corpuscle. The metabolic 
wastes pass from the blood through the thin walls of Bowman's 
capsule into a collecting tubule. The urinary wastes in the bird con- 
tain little liquid and pass on into the cloaca as semisolids, which are 
chiefly uric acid rather than urea. The urinary wastes dry and harden 


on contact with air and may be recognized as a whitish deposit on 
the fecal droppings. A bladder is not found in birds. 

Reproductive System. The reproductive system is considered 
further in Chapter 6, so that only its anatomy need be given here 
(Fig. 4-16). The two testes of the male lie forward of the kidney 
and immediately under the back. The sperm is produced in the testes 
and passes to the cloaca through the deferent ducts. There is no 
copulatory organ in most birds; a genital papilla serves as the end of 
the deferent ducts. In Ducks and some other birds (Ostrich, Casso- 
wary, Emu, Kiwi) a penis is present, but other birds copulate by 
direct contact of the cloaca of the male with that of the female. The 
sperm is long and cylindrical and terminates in a motile tail by which 
it propels itself (Fig. 6- 1 ). 

In the nonbrecding season, the ovary looks like a small mass of 
grapes, the mass being formed of undeveloped ova or eggs. Seasonally 
they develop by addition of yolk while within the ovary, from which 
they break out at daily or longer intervals to pass down the oviduct 
to the cloaca, from which the final egg is laid. 

Endocrine System. The animal body regulates its actions through 
the nervous and endocrine systems (Fig. 4*17). There are several 
endocrine glands within the body that secrete hormones into the 
blood stream to act as regulators of bodily functions through excita- 
tion or inhibition. These glands are thyroid, parathyroid, pituitary, 
gonads, adrenals, isles of Langerhans, and possibly the thyvms. 
Adrenal and thyroid glands have shown no significant sex difference 
(Hartman, 1946). 

Fig. 4-17. The endocrine system of the bird in its approximate posi- 
tion, (a) Isles of Langerhans, (b) gonads, (c) adrenals, (d) pituitary, 
(e) thymus, (f) parathyroid, (g) thyroid. 


The thyroid gland regulates metabolism; its secretions are also neces- 
sary for proper growth and sexual development. The parathyroids 
are two small bodies fastened to the lower part of the thyroids and 
concerned with calcium metabolism and perhaps with other functions. 
The pituitary (hypophysis) produces several secretions; those of the 
anterior pituitary regulate the other glands of the body and prob- 
ably also have some body-regulation functions of their own. The 
hormones produced by the posterior pituitary may have some meta- 
bolic regulatory function. The pituitary gland also regulates growth. 
The gonads serve as endocrine glands through hormones secreted by 
the interstitial cells in addition to their primary reproductive function. 

The adrenal (suprarenal) gland secretion, adrenalin, controls the 
glycogen supply of the blood, the heart rate, and perhaps the con- 
striction of blood vessels. The intimate connection between the 
adrenal gland and the nervous system permits nervous tension, as 
occurs in emergencies, to be transmitted quickly to the adrenals for 
immediate stimulation. The isles of Langerhans are cell masses within 
the pancreas that produce a hormone, insulin, which regulates carbo- 
hydrate metabolism. The thymus gland appears in the neck region of 
the embryo but becomes much reduced by hatching time, Its function 
is not known; it may actually be a lymph mass rather than a true 
endocrine gland. 

Table 4-3 gives some samples of the weights of various body 
organs and parts of the bird. 


Nervous System. The body of the bird has a well-developed 
nervous system. Birds respond to stimuli more rapidly, generally speak- 
ing, than do most other vertebrates. For all practical purposes, birds 
carry on much of their life by instinct, because their reasoning ability 
is low. Some writers have called birds "feathered automatons," but 
this statement pays little tribute to their intellectual capacity. Experi- 
ments to test their intellectual capacity have shown that among others, 
the Corvidae rank high, so that the expression "smart as an old Crow" 
does have some merit. The size of the brain varies with that of the 
bird and presumably with its general level of intelligence. That of 
the Crow weighs 9.3 grams, more than that of the larger Sandhill Crane 
(8.58 grams). Other brain weights range from about 0.564 grams in 
the Canary (15 grams body weight) to 42.11 grams in the Ostrich 
(123,000 grams body weight) (Quiring, 1951). 

The nervous system as a whole consists of sensory organs and the 
central and sympathetic (involuntary) nervous systems. The centra) 


nervous system has two parts, the brain and the spinal cord. The brain 
in reality is an enlargement of the forward end of the spinal cord. The 
great sense organs of sight and hearing and the seat of many other 
mental faculties are located in the brain. 

The brain itself consists of several parts (Fig. 4- 18), the rearward 
one of which is the wyelencephalon (medulla oblongata). It grades 
imperceptibly into the spinal cord. The next forward portion is the 
metencephalon, formed chiefly of the cerebellum, which is compara- 
tively large in birds. The r mesencephalon lies ahead of the meten- 
cephalon and controls vision through the optic lobe; it also concerns 

cerebrum ^, , .// \ . I ./. /^\ cerebellum 

Fig. 4-18. Brain of the bird (Rock Dove). The numbers indicate the 
roots of the corresponding cranial nerves. (The third is underneath.) 

itself with hearing. Still farther forward lies the diencephalon, which 
serves chiefly as a relay center. The front part of the brain, telen- 
cephalon or cerebrum, in many vertebrates handles largely the sense 
of smell. But the olfactory lobe is much reduced in modern birds in 
consequence of their poorly developed sense of smell. (Some birds, 
however, seem to use odor in choosing foods.) The sense of taste 
seems to be well developed in some birds and poor in others. 

The peripheral nervous system consists of nerves and nerve end- 
ings that gather sensory impressions. Nerve tracts along which they 
pass to the central nervous system are sometimes considered as part of 
the central nervous system and sometimes as part of the peripheral. 
Nerves connected with the brain are cranial nerves, those attached 
to the spinal cord, spinal nerves. 

There are twelve pairs of cranial nerves (Figs. 4- 18 and 4- 19) in 
birds (a thirteenth, the nervus terminalis is probably absent in birds) ; 



Fig. 4* 19. Function of the cranial nerves (indicated by Roman numer- 
als), (a) Hawk sees rodent, II. (b) Files to prey: sees rodent, II; focuses 
eye, III; moves eyes slightly, III, IV. (c) Tears food, V; moves food with 
tongue, VII; tasting, and salivation, VI, IX; swallows food, X, XII; moves 
neck, XL (d) Resting on perch: watches, III, IV, VI; listens, VIII. The 
olfactory nerve, I, probably plays no part in the feeding process. 

Table 4- 5 
The Cranial Nerves 




Source in 

Point of 





Olfactory bulb 





Retina of eye 





Kxtrinsic and in- 

trinsic eye 


Helps move eye 




Superior oblique 

eye muscle 

Moves jaw 




Jaw muscles 

Helps move eye 




Lateral rectus 


Taste, controls 




Taste buds, head 


muscles, mouth 






Mouth glands, 


Control of 











Pharynx, viscera, 



syrinx, trachea, 


Control of 


Spinal accessory 


Pharynx, viscera 

pharynx, neck, 


Control of 




Tongue muscles, 

tongue, syrinx 



these nerves are referred to by number. In the course of structural 
change during evolution of the body, they generally follow the muscle 
as originally controlled in their ancestors, irrespective of where the 
muscle may come to rest in modern birds (Table 4-5). 

The spinal nerves arise from the spinal cord and are therefore not 
directly connected to the brain. They are part of a complex organiza- 
tion through which the heart, lungs, digestive tract, blood vessels, and 
many other parts of the body are controlled "involuntarily." In the 
bird, the spinal nerves have far greater control over the body than 
in the mammal. This is illustrated by the rather violent muscular 
contortions following the simple farmyard practice of decapitating 
a Chicken. The saying "like a Chicken with its head cut off" in refer- 
ence to undirected human effort testifies also to the control of the 
body by the spinal cord in birds. 

There arc about fifteen to thirty pairs of spinal nerves in the bird. 
Those in the upper neck region form a cervical plexus that innervates 
the muscles and skin in the head and neck region. Those from the 
lower neck combine with the first dorsal nerves to innervate the 
muscles of the wings and breast region through a merger called the 
Irrachial plexus. The remaining dorsal nerves innervate the nearby 
region and contribute some inncrvation to the wings also. The nerves 
that control the hind limbs and tail region originate in the Iwnbosacral 
plexus (of three plexi, lumbar, sacral, and pudendal). 

Reception of stimuli from the outside is through the sense organs, 
which in the bird consist chiefly of the ears and eyes. Birds have 
many nerve endings throughout the skin, however, particularly near 
the base of many feather follicles. The whole body thus has a sense 
of touch; in a way, every feather may serve as an "organ of touch." 

The bird probably senses in varying degrees the same things 
humans do, such as heat, cold, pain, hunger, thirst, and kinesthetic 
senses. The impulse, when picked up by the proper nerve ending, 
passes to the brain where the actual recognition takes place. Scattered 
through the muscles and body organs will be found nerve endings for 
the maintenance of body operations and the reception of internal 
stimuli, such as feelings of hunger, pressure, muscle position, and the 
like. Temperature regulation occurs automatically by means of the 
therwotaxic nerve center in the corpus striatum of the cerebral hemi- 
spheres. Overheating or underheating results in setting in motion ap- 
propriate actions for heating (page 86) or cooling (page 87) the 
body as the need arises. Changes in insulation power of the feather 
covering play a role in temperature control (Moore, 1951). 

Vision. Birds possess very large eyes, often larger than the brain 
itself (Table 4-3). In fact, the 2-inch eye of the Ostrich is the largest 


that occurs in any land animal. The eye of a 2- or 3-pound Hawk or 
Owl, for example, may be as large as or even larger than that of man. 
A bony sclerotic ring (Fig. 4-20) may be present. The rapid accom- 
modation of the eye, perhaps accomplished by moving the lens as in 

Fig. 4-20. The sclerotic ring of the Great Horned Owl. 

a camera as well as by changing lens shape, necessitates considerable 
power (Fig. 4-21). A Prairie Falcon, for example, diving at high 
speed for a small target in the form of a rodent head projecting out 
of its burrow (page 170) must accommodate rapidly, for mistakes in 
the bird world are often fatal. So far as known, accommodation is 
faster in the bird than in other vertebrates. 

The retina has great resolving power. In the retinal foveas of a 
soaring Hawk, a million cones per square millimeter have been re- 
ported. The visual acuity of Hawks and Eagles exceeds man's by 
at least eight times (Walls, 1942). For purposes of adapting eyes to 
needs of the bird, three chief types occur (Fig. 4-21). 

Because the large eye occupies so much of the skull, great muscles 
would be needed to turn it like the roving eye of man. The bird has 
simplified matters by having a relatively stationary eye and using the 
existing neck muscles to turn the head and thereby to accomplish eye 
motion. In any event, the side position of the eye on the head gives a 
wide visual area perhaps three-quarters of a circle (Fig. 4-21). 

The eyes of most birds seem to have monocular (one eye at a 
time) vision to the side, though Owls and some others have both eyes 
forward for binocular vision. The latter permits special facility in 
judging distances. It seems evident that birds can judge distance very 
well and that they are able to adjust from a bill-length focus to an 
infinite one as rapidly as need be. Most birds have two foveas in each 
eye, one for looking forward and the other for looking to the side. 
In the Swifts and Swallows, a third one appears; in Owls there is but 
one. Because a bird presumably can use but one eye at a time in 
monocular vision, it appears able to ignore or suppress images in the 
other eye or another fovea. The temporal foveas, however, give 
binocular vision in the center of the circle of vision (Fig. 4-21). 



Two eyelids, the upper and lower, and a nictitating membrane 
cover the eye. The upper eyelid moves little and a bird usually closes 
its eye by drawing up the lower lid. The nictitating membrane can 
be drawn across the eye from its lower nasal position upward and 
rearward to moisten and clean the cornea. The moisture comes from 
lachrymal glands under the upper eyelids. The Harderian glands 
under the nictitating membrane also supply lubrication. 

Nocturnal birds, such as some Owls, have a sensitivity to low 
light intensities ten to a hundred times greater than that of man. 
Diurnal Owls, on the other hand, may have no better or have even 

Scleral ossicle 
Annular p 
Ora termmalis 

Fig. 4*21. (a) "Flat" eye common to many birds, (b) "tubular" eye 
found in Owls, (c) "globose" eye of Red-tailed Hawk with parts labeled, 
(d) monocular vision through use of the central fovea, (e) binocular 
vision through use of two temporal foveas. (Prepared by Rex Lord.) 


poorer night vision than man. Under favorable conditions the Barred, 
Long-eared, and Barn Owls can see to approach prey from a distance 
of 6 feet or more under an illumination as low as 0.000,000,73 foot- 
candles (Dice, 1945). This would be the amount of illumination pro- 
duced by a "standard candle" at a distance of 1,170 feet. Owls may 
see objects with difficulty down to 0.000,000,15 foot-candles, which 
would be equivalent to a standard candle at 2,582 feet. The light on 
a cloudy, moonless night may be 0.000,4 foot-candles, but might fall 
to 0.000,000,4 on the forest floor in deciduous woods. Hence, prey 
in the deep shadows of a dark night may not be visible to the best 
of Owl eyes. 

Birds perceive color somewhat as in the human eye, though perhaps 
with less sensitivity at the blue end of the spectrum and more sensi- 
tivity at the red end. The red and yellow oil droplets in the retina 
act as color filters. The yellow serves during most of the day and the 
red in the morning and evening when they are exposed to the Ray- 
leigh effect of light scattering. Birds that rise at dawn or earlier, like 
most Songbirds, have about 20 per cent red droplets; Hawks have 
about 10 per cent; and Swallows and Swifts, 3 to 5 per cent. Birds 
that must face the glare over the water have increased numbers of red 
droplets; thus, the European Kingfisher has 60 per cent, the Water 
Ouzel (Dipper), 24 per cent; Ducks, 20 per cent; and Herons, 20 per 
cent. There appears to be no relationship of plumage color and color 
of oil droplets, save for the fact that green oil droplets have been re- 
ported in some green Parrots (Walls, 1942). 

Diurnal eyes, such as those found in common daytime birds, are 
rich in retinal cones. These possess red, orange, yellow, and colorless 
oil droplets. Crepuscular eyes, such as those in the Nighthawk, have 
a higher ratio of rods to cones. They have few or no red and orange 
droplets, some yellow droplets, but mostly colorless ones. Nocturnal 
birds have a high ratio of cones to rods for increased visual sensitivity 
at night, but enough rods are present for good daytime vision. The oil 
droplets in such birds may be wholly colorless ones. 

Hearing. Though the hearing of a bird is relatively good, reaction 
to sounds may depend much upon sight. Thus, an observer in a blind 
may make many sounds clearly audible to, but not disturbing to, a 
bird. Most birds, however, react to sharp sounds whether with or 
without motion. The hearing range of birds will be discussed further 
in Chapter 17. 

The ear of the bird consists of a middle and inner ear formed of 
semicircular canals, vestibule, tympanum, cochlea, sacculus, utriculus, 
and associated nerve tissue (Fig. 4-22). The sound reaches the inner 
car through the middle ear by an opening in the skull covered by 










Fig. 4-22. The semicircular canals maintain balance as in the Toiohee 
(Pipilo). (a) Bony canal, (b) membranous canal, (c) crista acustica, 
(d) cupola. (From William J. Beecher, "A Possible Navigation Sense in 
the Ear of Birds," American A/Iidland-Naturalist, 46(1951):368.) 



Fig. 4*23. Normal attitude In birds. The "rest position" is one wherein 
the external canal is horizontal. Various species attain it in rest or flight 
by different elevation or depression of the bill. (From William J. Beechcr, 
"A Possible Navigation Sense in the Ear of Birds" American Midland- 
Naturalist, 46(1951 ):374.) 


special feathers. There is no external ear as in mammals, though an 
"ear conch" in some birds, particularly Owls, may be an aid to hear- 
ing and locating night sounds. The ear conch tends to be larger in 
Owls of cold climates, apparently an exception to the Allen rule 
(Kelso, 1940; see page 185). 

Sounds pass to the tympanum of the middle ear and thence along 
the rodlike colwmlla into the inner car where as vibrations they are 
picked up by the hairlike endings of the auditory nerve for transmis- 
sion as nerve impulses to the auditory center of the brain along the 
eighth cranial nerve. 

The ear serves also as the gravity-balancing organ of the body (Fig. 
4-23). Equilibrium is maintained by a fluid (endolympb) in the 
semicircular canals, which in shifting about stimulates the hair cells. 
Other parts of the inner ear may participate in detecting movement, 
such as of changes in speed or direction. A bird in flight or at rest 
holds its head in a normal position when the balancing organ functions 
properly. Injury or malfunctioning causes characteristic abnormal be- 
havior. Birds orient themselves to wind in part at least by the balance 
of air pressure between the two ears. 


BEDDARD, FRANK K., The Structure and Classification of Birds. Boston: Longmans, 
Green & Co., 1898. 

BRADLEY, O. C., and TOM GRAHAME, Structure of the Fowl. Edinburgh, Scotland: 
Oliver & Boyd, 1950. 

COUES, ELLIOTT, Key to North American Birds. Boston: The Page Co., 1912. 

FISHER, HARVEY I., "The Occurrence of Vestigial Claws on the Wings of Birds," 
American Midland-Naturalist, 23(1940):234-243. 

FISHER, HARVEY I., "Adaptations and Comparative Anatomy of the Locomotive Ap- 
paratus of New World Vultures," American Midland-Naturalist, 35 (1946): 545-727 

HUDSON, GEORGE ELFORD, "Studies on the Muscles of the Pelvic Appendage of Birds,'* 
American Midland-Naturalist, 18 (1937): 100-108. 

HYMAN, LIBBIE HENRIETTA, Comparative Vertebrate Anatomy. Chicago: University 

of Chicago Press, 1942. 
* NEWTON, ALFRED, A Dictionary of Birds. London: Adam & Charles Black, 1893-1896. 

QUIRING, DANIEL P., Functional Anatomy of the Vertebrates. New York: McGraw- 
Hill Book Co., Inc., 1950. 

SHUFELDT, R. W., The Myology of the Raven (Corvus corax sinuatus). New York: 
The Macmillan Co., 1890. 

SHUFELDP, R. W., Osteology of Birds. New York State Museum, Bulletin No. 130, 

THOMSON, J. ARTHUR, The Biology of Birds. New York: The Macmillan Co., 1923. 

WALLS, G. L., The Vertebrate Eye. Bloomfield Hills, Mich.: Cranbrook Institute 
of Science, 1942. 

WEICHERT, CHARLES K., Anatomy of the Cbor dates. New York: McGraw-Hill Book 

Co., Inc., 1951. 

*YOUNG, J. Z., The Life of the Vertebrates. Cambridge, England: Clarendon Press, 


The Basis of Life 

Fundamentally, about all things as we know them, whether those 
of our own making or those of nature, rest upon transfer of energy 
at some time or other. Though it may not be visible in man's short 
time on earth, few things of the universe are so static as not to be part 
of some energy transfer system. In its way too, the bird is part of an 
energy transfer process, for the basis of life is the energy liberated in 
the body cells. In a broad sense, physiology deals with the genera- 
tion and use of energy. 

Living things consume energy, and the bird is no exception. Even 
when a bird is at rest, internal tension, life processes, and temperature 
demands require energy, and all combine to drain the energy resources 
of the bird. Living things no doubt differ in their efficiency in using 
energy, just as man-made machines may differ also. The more efficient 
users of energy clearly have an advantage over others. As said else- 
where, it is a principle of thermodynamics that when two systems 
compete in using the same energy resource, the more efficient prospers 
at the expense of the less efficient. The maximum efficiency of bird 
muscle is reported to be about 40 to 50 per cent (and possibly more). 
This compares better than favorably with the 35 to 40 per cent effi- 
ciency of diesel engines. 

Generating energy to maintain life is the operational role of the 
"protoplasm" that makes up the animal body. Succinctly speaking, this 
job of "protoplasm" is just to be alive. "Protoplasm" itself consists of 
many complex chemical compounds, chief among which are proteins, 
fats, and carbohydrates-, but more than half, sometimes as much as 90 
per cent, is water. In addition, "protoplasm" contains many minerals 
and other elements (generally in small quantities), such as carbon, 
oxygen, hydrogen, phosphorus, sulphur, sodium, potassium, calcium, 
magnesium, chlorine, iron, or copper. 




Oxidation. The release of heat and energy results from biological 
oxidation, either aerobic (involving atmospheric oxygen, the usual 
process of oxidation) or anaerobic (involving no atmospheric oxy- 
gen). Anaerobic oxidation produces in the body many times the im- 
mediate energy of normal oxidation and functions in emergencies. 
Yet it results in the accumulation of lactic acid and other acids in the 
muscles. It results also in an "oxygen debt"; hence, it cannot long be 
sustained without fatigue. 

The oxygen used directly in aerobic oxidation and that used in- 
directly in anaerobic oxidation comes to the tissues chiefly through 
the hemoglobin of the blood. The hemoglobin content of bird blood 
seems to vary between about 10 and 18 per cent. Diving birds have 
reserves in the oxyhemoglobin and oxymyoglobm? so that they are 
capable of long submergence, though species vary in this capacity 
(Schorger, 1947). Birds accustomed to high altitudes have a higher 
erythrocyte concentration than those of lower altitudes, and their 
blood generally has a higher oxygen affinity. Birds of higher altitudes 
also have relatively larger hearts (Hartman, 1955). Both the liver and 
spleen store quantities of blood rich in oxygen that can be put back 
into circulation. It may be also that birds ascending to high altitudes 
or diving deep into the water can shift blood from tissues less sensi- 
tive to oxygen deficits to the muscles being used or to the nervous 
system, which is particularly sensitive. 

Body Temperature. The normal diet (daily) temperature of the 
diurnal bird rises to a maximum in the late afternoon, after which it 
declines to a low early in the morning. The daily temperature rhythm 
of nocturnal birds (like most Owls) is reversed and reaches its high 
late at night and its low sometime during the day. Temperature 
change varies also with activity. Its range varies with size, small birds 
tending to have a greater variation than large ones. A daily variation 
of 10.6 F. has been reported for the American Robin, 10.0 F. for 
the Song Sparrow, but only 1.8 F. for the Domestic Duck. 

The average temperature of the female bird usually exceeds that 
of the male by a small amount, though exceptions have been reported 
for some species (as among the Ardeidae and Phalaropodidae). 

The newly hatched altricial bird is essentially cold-blooded, like 
its reptilian relatives (Fig. 1-9). Development of temperature control 
takes some time (Kendeigh, 1939). The newly hatched youngster 
depends upon brooding by the adult for maintenance of its body 
temperature, which fluctuates with that of the air when the parent is 


away. But the body temperature becomes about that of the adult by 
the time the young leave the nest. In precocial birds, however, tem- 
perature regulation is better and the young are more nearly able to 
maintain normal body temperature. In adults, body temperature may 
rise and fall with activity and rest but not with hot or cold weather. 

Wetmore (1921) summarized findings on the temperature of 327 
species of fifty families and added a table of data taken from the liter- 
ature. Generally speaking, the increase in body temperature from low 
to high follows the taxonomic position, the more highly placed birds 
having the higher temperatures. Presumably, the placement of birds 
reflects somewhat their relative development which in turn would 
seem to reflect relative efficiency. The higher temperature of the 
higher-ranked birds would perhaps increase their efficiency as energy 
consumers. The normal temperature varies from about 103 F. in 
Grebes and Pelicans to about 109 F. in perching birds (Table 5-1). 

Table 5* I 
Reported Body Temperatures of Various Species of Birds 


Body . Body 
Temperature pecies Temperature 


... 101.8 

Horned Lark 

. 109.4 


. . . 100.0 

American Crow 

. 107.9 

Penguin . . 


American Raven . . 


Pied-billed Grebe 


Carolina Chickadee . 


A Ibatross 


White-breasted Nuthatch... 

. 107.7 

Brown Pelican 


Canada Goose 

. . . 106.9 

American Robin 

. 109.8 



Magnolia Warbler . . 


Turkey Vulture 

103 8 

Western Meadowlark 



. . . 106.6 


. 108.1 

Mourning Dove 

. 109.0 


. 109.3 

Great Horned Owl 

. 103.8 

Chipping Sparrow 

. 107.4 

Eastern Nighthawk 


Song Sparrow 

. 109.1 

Downy Woodpecker 


Source: Alexander Wetmore, A Study of Body Temperature of Birds , Smithsonian 
Miscellaneous Collections, Vol. 72, No. 12. 

The lowest "normal" temperature seems to be in the Western Grebe 
(101.3 F.) and the highest in the Western Wood Pewee (112.7 F.). 
Hummingbirds show low temperatures, but their small size in respect 
sto thermometers may be responsible. Yet it seems that the temperature 
of the Hummingbird at night drops almost to that of the air as a 
means of conserving energy resources (Pearson, 1953). 

Metabolic Rate. The metabolic rate of birds varies among species; 
in general, it varies with the two-thirds power of the body weight. 



Table 5- 2 

Body Size as Related to Four Hours of Heat Production 
(From various sources) 




per 1,000 

Calories per 
Square Meter 
of Surface 






"Sparrow" ... 

. . . . 22.5 


































a- 07 


E 6 
3 0.5 


-30 -20 

-10 +10 +20 +30 


Fig. 5 I . The energy intake of the House Sparrow increases with de- 
crease in temperature and decreases with increase in temperature. The 
liberation of productive energy, however, increases with rise in tempera- 
ture and consequent lower demands on energy of the body for warmth. 
(After S. Charles Kendeigh, "Effect of Temperature and Season on 
Energy Resources of the English Sparrow" Auk, 66(1949):111.) 



Larger birds thus have a proportionately lower metabolic rate than 
smaller ones (page 436). In a like manner, a larger bird produces less 
heat for its body weight than a small one, though the amount pro- 
duced per unit of surface is about the same, as shown in Table 5-2. 

In addition to surface area, it seems probable that shape of the 
surface influences radiation in accord with Lambert's Law. Perhaps 
other factors enter into the radiation rate. 

It will be seen in the list given (Table 5-2) that though the calories 
produced per thousand grams of body declined from 301 for the 
Canary of 16.3 grams weight to 54 for the Goose of 5,000 grams body 
weight, the calories per square meter of body surface changed rela- 
tively little. It was highest (930) for the Goose and lowest (609) for 
the Dove. The importance of the Bcrgmann Rule (page 185), as well 
as its operation, is apparent in the light of reduction of heat produced 
per unit of weight with size increase. 













Potential energy intake 

Existence energy 

Productive energy 







Fig. 5*2. The energy balance of the House Sparrow varies through- 
out the year. (After S. Charles Kendeigh, "Effect of Temperature and 
Season on Energy Resources of the English Sparrow," Auk, 66(1949): 



The metabolism of the adult bird increases with decrease of air 
temperature. More energy from food is needed at low temperatures 
than at higher ones (Fig. 5-1). The energy required for the House 
Sparrow at 40 F. below zero is more than triple that required at 
100 F. above zero. Just keeping alive at low temperatures obviously 
is equivalent to a considerable expenditure of energy. At low air 
temperatures, the body temperature remains normal so long as oxida- 
tion can be carried on at high enough rates; failing that, the body 
temperature declines; if prolonged, it may result in death. Fig. 5-2 

Zone of thermal 




Fig. 5 3. Relationship of the metabolic rate in resting homoiothermous 
animals to environmental temperatures. (After Douglas H. K. Lee and 
Ralph W. Phillips, "Assessment of the Adaptability of Livestock to Cli- 
matic Stress" Journal of Animal Science, 1 ( 1948 ):3 9 1-425.) 

illustrates the variations in energy balance of the House Sparrow 
during the year. At the opposite extreme (high air temperature) the 
body maintains normal temperature by ventilation and water evapora- 
tion in the respiratory system (peripheral circulation in birds is poor) . 
Inability to eliminate heat fast enough means a rise of body tem- 
perature and death if prolonged above that normal for the species. 
Thus both high and low temperatures, if prolonged, may be fatal 
(Fig. 5-3). 

Heart, Respiration Rate, and Cooling. The problem in con- 
trolling excess heat is really one of heat dissipation. Birds accomplish 
this largely by increasing the breathing rate, which at rest on a mod- 



erate day may be low but which may rise to 200 per minute at high 
temperatures. Respiration and heart rates per minute reported for 
various birds are listed in Table 5-3. 

Table 5 -3 
Heart and Respiration Rates Per Minute 


Heart Rate Respiration Rate 

Condor - 6 

Pelican - 4 

"Wild Duck" 185 

Canary 1,000 100 

Chicken 304;312 20;12 

Turkey 9} 14 

Pigeon 244 26 

Duck - 22 

Goose - 16 

House Wren - 1 1 2;92 

American Robin - 45 

Chipping Sparrow - 83 

Downy Woodpecker - 131 

Falcon 347 

Hairy Woodpecker - 120 

House Sparrow 800 94;104;140 

Blue Jay - 69;78 

Baltimore Oriole - 107 

Starling - 84 

Bronzed Crackle - 55 

Red-shouldered Hawk - 34 

Barred Owl - 142 

Barn Owl - 184 

Great Horned Owl - 223 

Bob-white 322;504;462 64;74,100;142 

Ring-necked Pheasant 252;298 44;41;50 

Hungarian Partridge 192;264 44;52 

Scaled Quail - 84;170 

Plumed Quail 402 89;92 

Source: William H. Long, The Heat Production and Muscular Activity o) Two 
Strains of Wild Turkeys, Pennsylvania Game Commission, Research Bulletin No. 2. 

Under stress of heat, a bird's breathing rate increases oftentimes 
to the point of actual visible panting. The Domestic Chicken is re- 
ported to have a mean body temperature of 106.2 F. and to show 
rapid breathing at a body temperature of 107.5 F., panting at 108 
F., agitation at 1 1 1 F., and gasping at 1 1 3 F. We should expect wild 
birds to have parallel behavior. The breathing rate of the House Spar- 
row has been reported to be as low as 94 per minute at about 55 F. 
below zero and 200 per minute at 125 F. above. Some seasonal 
variation may occur (Fig. 5-4). Panting helps to increase ventilation 
of the body interior through the lungs and air sacs. At high air tern- 


| 600 


a! 550 


< 450 



80 LJ 

70 o 


90 70 43 

Fig. 5*4. Average standard heart and breathing rates of the Black- 
capped Chickadee at air temperatures of 90 F. and 43 F. during winter 
and spring. (After Eugene P. Oduw, "So?/ie Physiological Variations in 
the Black-capped Chickadee," Wilson Bulletin, 55(1943):185.) 

peratures, therefore, most of the heat loss occurs through evaporation, 
for rapid breathing increases evaporation in the lungs, air sacs, and 
air passages. It has been suggested that evaporation from the inner 
surface of the bill pouch while "panting" may aid cooling in the 
Pelican and Nighthawk (Fig. 5-5). At low temperatures, radiation 
losses are more important than evaporation losses. 




| 41.5 










10 15 



Fig. 5 - 5. Effect of gular fluttering on body temperature as meas- 
ured at the cloaca of a bird placed in the sun. Zero time is the moment 
'when placed in the sun. (After Raymond B. Coivles and William R. 
Daivson, "A Cooling Mechanism of the Texas Night hawk" Condor, 53 


The air temperature at which production needs and radiation needs 
are neutral (in effect, when the bird feels neither cold nor warm) 
seems to be about 78 to 86 F. for land birds. Ground inhabitants of 
the forest and brush have lower thermo-neutral temperatures than 
those of the open, and birds of cooler climates have lower thermo- 
neutral temperatures than those of warm ones. 

Heat Loss. In nature, birds protect themselves against heat loss in 
a variety of ways. They get into protected areas and fluff up the 
feathers, for example, which increases the insulation value of the 
feathers. The bird pulls in its extremities and takes on a humped-up 
look. Several may huddle together for warmth as well as protection. 
Homoiothermous animals in general increase body heat by greater 
muscular activity and by greater outpouring of hormones from the 
thyroid and adrenal glands to stimulate heat production, as may be 
seen in Table 5-4. The air temperature, where conservation of heat 
by insulation becomes insufficient, varies with species. For tropical 
birds, it seems to be about 20 to 23 C. For the Canada Jay, it may 
be -10 C. and for the Glaucous Gull -40 C. 

Table 5 -4 
Thermal Response of Endocrine Glands 


Response to cold 

Response to heat 

Anterior pituitary 

Adrenal medulla 

Increased thyrotropic hor- 
Increased thyroxin 
Increased adrenalin 

Decreased thyrotropic hor- 
Decreased thyroxin 
No change unless under 
great stress 

Adrenal cortex Increased kctostcroids 

Source: Douglas H. K. Lee and Ralph W. Phillips, "Assessment of the Adaptability 
of Livestock to Climatic Stress," Journal of Animal Science, 7 (1948): 391-425. 


Nutrition. Much can be said about the diet of domestic or confined 
animals, as well as of man himself. But little is known about the physi- 
ology of the nutritional aspects of wild birds. On the basis of studies, 
it seems apparent that the Domestic Chicken, if given a free selection 
of a wide range of foods encompassing the needs of a balanced diet, 
will balance its rations satisfactorily. It hardly seems that birds in the 
wild would do less; but we cannot be sure that birds confronted with 
a limited dietary (as in winter) will be able to balance their diets with 
the foods at hand (Chapter 23). 


It seems entirely likely that birds, at least many birds, can syn- 
thesize sufficient vitamin C for their needs. Other vitamins may be 
available in adequate amounts in nature during the growing season 
and perhaps throughout the year. Whether the needed vitamin D, for 
example, can be carried in the body from summer (when ultraviolet 
rays penetrate to the bird environment in the North) through the 
winter (when ultraviolet rays may not reach the surface in the 

North) is not known for wild birds. 


Weight Variations. The weight of the bird may give some indica- 
tion of its physiological condition (page 401), but we have as yet 
few standards of comparison (Fig. 5-6). Birds are highly sensitive 

614.7 g. 534.2 g. 456.6 g. 370.6 g. 307.6 g. 

Fig. 5 6. Breakdown of the body during starvation shows hi the reduc- 
tion of breast muscle in the Coot. The weights in grams of the birds 
(all wales) are shown. (After Milton B. Traiitman, William E. Bills, and 
Edward L. Wickliff, u Winter Losses ]rom Starvation and Exposure of 
Waterfowl and Upland Game Birds in Ohio and Other Northern States" 
Wilson Bulletin, 51(1939):93.) 

to environmental influence, and considerable variation occurs among 
the individuals of a species and between different periods of time for 
the same individual (Nice, 1938). In general, males are heavier than 
females, but departures from such a generality may occur. The 
females of raptorial birds consistently tend to outweigh the males; and 
young birds that lay on substantial fat deposits may be heavier tem- 
porarily than either parent. 

Marked daily (diel) variations in weight may occur. The greatest 
weight is reached late in the afternoon and the lowest late in the 
night. The weight will increase or decrease with food intake, as one 
would properly expect. It varies also with water intake and loss. The 
diel variation in Passerine birds may reach 12 per cent or more of the 
mean daily weight. The food eaten probably accounts for much of 
this, as may be seen in the chapter on food habits, though small birds 
eat proportionately more than large ones (Fig. 5-7). 

Weight may vary seasonally, for short periods, and often with air 
temperature. Weight loss may be great with a decline of temperature, 
which probably reflects energy consumption in keeping the body 
warm. Birds reach their lowest weights generally in midsummer, how- 
ever, and highest in midwinter or later. Variations in body fat largely 
account for this. 



Among some migratory species, the maximum weight is reached 
immediately before migration in consequence of heavy fat deposition 
resulting from increased metabolism under the influence of the endo- 
crine system as controlled by the pituitary gland (Wolfson, 1945). 
This fat deposition can occur with suddenness, perhaps within days 
but certainly within a fortnight or so, though it may be prolonged 
for several weeks. It seems evident that the laying on of fat stores 
energy for migration use, just as a hibernating mammal lays on fat be- 
fore hibernation. Day length seems to be the controlling factor which 

100 200 

300 400 500 600 

700 800 900 

Fig. 5 -7. The relative ]ood intake decreases 'with increase h? weight of 
the bird. Small birds eat as much as 30 per cent of the body weight daily, 
while large ones may eat only 3 per cent. (Data after Margaret Morse 
Nice, "The Biological Significance of Body Weight" Bird-Banding, 

induces pituitary changes and fat deposition, not the warming of 
spring or cooling of fall. But nonmigratory birds, even though of 
the same subspecies as other populations which migrate, may not lay 
on fat, even though they are living in the same place and under the 
same environmental influences. This marks an evident difference in 
their response to the environment. Female birds take longer for such 
effects, and lay on fat later, which is in accord with their general 
practice of migrating later than the males. 

The increase of day length (photo period) markedly influences the 
pituitary, and careful studies demonstrate that light itself influences 
the pituitary gland after reception through the eye (and possibly to 
some extent through the skin). Red light has proved most effective 
and green light the least effective. The stimulated pituitary gland, 
primarily the anterior lobe, in turn stimulates metabolism (as for the 
production of fat) and particularly the gonads. The gonads increase 
in size and spermato gene sis and oogenesis take place with increase in 
day length. Improper diet may adversely affect the influence of light, 
though such perhaps occurs infrequently in nature. 


Minerals. The bird body needs many minerals, such as calcium 
for bone growth or egg shells (page 442). Most minerals are needed 
in small or trace amounts. Field observations indicate that Doves, 
Crossbills, Pine Grosbeaks, Purple Finches, Evening Grosbeaks, and 
other species regularly resort to "salt licks." Analysis of several such 
licks in the West shows that the water-soluble salts present include 
chlorides, sulphates, hydrocarbonates, and carbonates of sodium. 
Some magnesium and calcium salts may also be present. 

Water Needs. Water needs, like the need for food, play so great 
a role in the life of birds that their importance should never be out 
of mind. The water needs of desert birds are particularly pressing, 
and the gathering of birds at a desert oasis readily testifies to this fact. 
The sources o.f water for birds and mammals alike are the same in 
deserts and humid regions: surface water, dew, succulent vegetation, 
sap, animal foods, and metabolic water (Vorhics, 1945). In hot cli- 
mates, much moisture may be needed for cooling the body by evap- 
oration in the lungs, air sacs, and gular membranes. 

Many birds drink water at frequent intervals, almost daily in sum- 
mer for White-winged Doves, Valley Quail, and Turkeys. But many 
birds of dry habitats clearly have no opportunity to drink. Yet the 
evident relish with which birds come to water at a feeding station 
testifies to its attractiveness. Birds on dry range appear to thrive 
better when moist foods are present, even though they may be able 
to survive on dry foods. Some may come into breeding condition 
earlier and more vigorously when an adequate supply of moisture is 

Most birds drink by dipping the bill in the water, elevating the 
head, and allowing it to run down the throat, but Doves drink with- 
out raising the .head. 

Marine birds often have no opportunity for obtaining fresh water 
for long periods of time. Some birds very clearly drink sea water, 
but others are made ill by it. The Adelie Penguin, for example, during 
the period of courtship ashore in the Antarctic spring, eats snow, a 
water form nearly as pure as distilled water. Yet later it drinks salt- 
laden sea water. Gulls, Waterfowl, and many other birds seem able 
to use both fresh and salt water. The salt-secreting ability of the 
kidneys appears particularly good in marine birds using sea water, 
but of this we are woefully ignorant (Murphy, 1936). 

Humidity. Atmospheric humidity influences the physiological 
well-being of birds, especially during hot weather. For some, the high 
humidity may well be # part of the environment to which they have 
become adjusted. But others, 'particularly in the mid-latitudes where 


most people interested in birds live, may not have become completely 
adjusted to high humidity. In hot weather, it retards evaporation and 
the consequent cooling of the body. In cold weather, it increases 
the conductivity of air and thereby the loss of heat. Paradoxical 
though it may seem, high humidity may be unfavorable in both hot 
and cold weather. 

The egg also requires a rather fixed humidity for best development 
of the embryo. A humidity greater than 50 per cent may be less fa- 
vorable than a lower one; it may be harmful. Low humidity, on the 
other hand, may cause excessive drying of the egg and may also cause 
some metabolic distress in the embryo. The eggs of water birds gen- 
erally require a higher humidity for hatching successfully than do 
those of land birds. 

Sleep. The physiological relaxation attendant upon sleep is little 
known for birds except in a most general way. The temperature, 
heart rate, and respiratory rate drop with night-time inactivity. But 
the mitotic rate, which measures the rate of repair or growth, in- 
creases. The eyes probably serve as the means through which darkness 
influences the body. Bodily fatigue also probably acts in coordination 
with the eyes and with other organs. Some birds seem so lacking in 
nervous response on their roosts as to be almost unaware of sounds 
and other disturbances (Fig. 11-3). Others seem rather easily dis- 


Starvation. Differences in survival time of starving birds may in 
part account for the inability of many birds to remain through the 
northern winter, though this may in itself be correlated with their 
life habit of absenting themselves from cold country in winter. As a 
general principle, well-fed birds are immune from normal adverse 
effects of cold in their native range. But a bird insufficiently fed, one 
with insufficient bodily reserves, or one unduly exposed may not be 
able to survive cold for any length of time. Thus, a bird going to a 
cold roost on an empty stomach may perish if the air temperature falls. 
Nine to 14 hours may be the maximum survival time at low tempera- 
ture for a common migratory Passerine bird with an empty stomach. 
Hence, the northward wintering of many birds becomes impossible 
during the long nights and short days of winter in higher latitudes. 

The ability to withstand cold nights, especially when hungry, va- 
ries among species and individuals (Kendeigh, W 1945c). To survive at 
low winter temperatures, most small birds must feed daily not only 


to supply energy used during the day but also to restore energy used 
the night before and to lay up energy for the night to come. In addi- 
tion, resident and wintering species in cold regions are better able to 
withstand such stresses than are migrants that move out. Thus the 
Slate-colored Junco and Tree Sparrow are better able to survive cold 
and hunger than the White-throated and White-crowned Sparrows. 
Conversely, the more southerly birds can survive higher summer tem- 
peratures better than can northern and Arctic ones. 

It seems well established that the flock roosting habit of the BoB- 
white covey (in a circle on the ground) has a survival value. A de- 
pleted covey is reported unable to survive cold night temperatures so 
well as a normal covey of a dozen birds, which may be a factor partly 
helping to explain why depleted coveys merge with others. The Bob- 
white covey in the North averages about two birds more than the 
covey of the South, evidently as an adjustment to greater cold of the 
North (page 239). 

Survival. All of a bird's energy is derived from the food it eats. 
To keep alive, the body must have a flow of energy from the outside. 
Interruption of this flow means that the body must call upon stored 
reserves, which it consumes until death ends the process. Experimental 
exposure to temperatures lower than usual for the species indicates 
that survival time for the House Sparrow without food declines at the 
rate of one-half hour for each two degrees of drop in temperature 
(Kendeigh, 1945, 1949). The presence of fat and other stored re- 
sources increases resistance. Also, the larger and more robust the 
bird, the longer the survival time under starvation. The Turkey, for 
example, is about fifty times larger than the Bob-white, and its sur- 
vival time is about five times as long. The Bob-white in turn is about 
six times the size of the White-crowned Sparrow and has a reported 
survival time of about three times as long. But there are no doubt 
many differences among species, just as there are among individuals. 
These differences may result from differing amounts of stored fat and 
other energy sources and the rate at which they are utilized. 

Of 136 House Sparrows picked up after a severe storm, 72 revived 
and 64 perished. In body measurements, those that perished varied 
most from the average of the type. It has therefore been suggested 
that those individuals which depart most from the average (which may 
be the "ideal" for the species) are most vulnerable to destruction when 
conditions become unfavorable (Bumpus, 1899). It has been shown, 
however, that in a large series of birds of the same sex and species, a 
natural variation of 15 to 20 per cent occurs in general size. Parts of 
the body may even vary independently of each other in a range of 15 
to 20 per cent. 



BALDWIN, S. PRENTISS, and S. CHARLES KENDEIGH, Physiology of the Temperature of 

Birds. Science Publications, Cleveland Museum of Natural History, 3(1932). 
BALDWIN, S. PRENTISS, and S. CHARLES KENDEIGH, "Variations in Weight of Birds," 

Auk, 55(1938):416-467. 
BISSONNEITE, THOMAS HUME, "Experimental Control of Sexual Photo-Periodicity in 

Animals and Possible Application to Wild-Life Management," Journal of Wildlife 

Management, 2 (1938): 104-1 18. 
BRODY, SAMUEL, Wio energetics and Growth. New York: Reinhold Publishing Corp., 

GERSTELL, RICHARD, and WILLIAM H. LONG, Physiological Variations in Wild Turkeys 

and Their Significance in Management. Pennsylvania Game Commission, Research 

Bulletin No. 2, 1939. 
KENDEIGH, S. CHARLES, The Role of Environment in the Life of the Birds. Ecological 

Monographs, 4 (1934): 299-417. 
PROSSER, C. LADD (ed.), Comparative Animal Physiology. Philadelphia: W. B. Saundcrs 

Co., 1950. 

STURKIE, PAUL D., Avian Physiology. Ithaca, N. Y.: Comstock Publishing Assoc., 1954. 
WETMORE, ALEXANDER, A Study of the Body Temperature of Birds. Smithsonian 

Miscellaneous Collections, 72(1921): No. 12. 
WOLKSON, ALBERT, "The Role of the Pituitary, Fat Deposition, and Body Weight in 

Bird Migration," Condor, 47 (1945): 95-127. 
WOLKSON, ALBERT, "Day Length, Migration, and Breeding Cycle in Birds," Scientific 

Monthly, 74 (1952): 191-200. 


Egg and Embryo 

Reproduction in birds depends upon the satisfactory performance 
of the reproductive system (genital system) and body functions as- 
sociated with breeding. The major associated functions (including 
breeding behavior) will be treated in their appropriate places, partic- 
ularly in Chapters 18 and 19. This chapter will therefore be devoted 
largely to the subject of egg production and development before 


Gonads. The genital system of birds consists of gonads (testes 
in males and ovaries in females) and ducts for transfer of the ova and 
spermatozoa to the outside (Fig. 6-1). 


Fig. 6-1. Examples of avian sperm, (a) Domestic Chicken, (b) Tyrant 
Flycatcher, (c) Domestic Pigeon, and (d) Sheldrake. 

Male. The testes of the male are paired organs producing the 
spermatozoa. The right testis may bulk larger than the left, which 
itself may be longer and narrower than the right one. During the 
breeding season, the testes expand in size, often many times larger 
than when quiescent during the nonbreeding season. The expansion 
occurs in the testicular tissue, in the number, size, and development 
of the spermatogonm, and in the interstitial cells. These latter cells 
function as the endocrine portion of the testes and produce testos- 




terone, which governs the manifestations and behavior associated with 
sex and reproduction. 

Spermatozoa develop in the semini] 'erous tubules and pass from the 
testes through efferent ducts to the epididymis, and from that to the 
sperm duct (vas deferens), through which they pass to the cloaca. In 
some breeding Passerines, the cloaca bulges out in a cloacal protuber- 




vas deferens 


Fig. 6 -2. The male urogenital system consists of the testes (the right 
one is on the left in the picture) and associated structures lor transfer of 
the spermatozoa to the outside. 

<wce owing to development of the seminal glomera. Near the cloacal 
end of each sperm duct, an enlargement forms the seminal vesicle, 
which stores sperm and produces secretions for carrying them (Fig. 

Female. The gonads of the female (Fig. 6-3) begin in the early 
smbryo as paired ovaries, but tfre right fails to develop so that only 
the left ovary becomes functional. In some members of Falconiformes 
(perhaps a half or more of all individuals), the right ovary persists, 
but it is not usually functional. The same occurs also in some Ducks 
md a few other species, and perhaps 5 per cent of common birds have 



a vestige of the right ovary. (A few cases have been reported of 
Domestic Chickens and Ducks with both ovaries and both oviducts 
functional.) The right oviduct dwindles in accord with its ovary. 
The retention of but a single ovary no doubt evolved as part of the 
process of weight reduction for an aerial life. The fact that one testis 









Fig. 6 3. Reproductive organs of the Domestic Chicken. A portion 
of the oviduct has been cut away to show a descending ovum. 

in the male is smaller than the other, sometimes markedly so, suggests 
a possible trend in the same direction in the male, though perhaps 
not so rapid as in the female because of the already small size of the 
testes themselves. This difference may, however, be merely lack of 
symmetry, for a similar difference occurs in other animals. 

Breeding Rhythm. The gonads increase in size seasonally in prep- 
aration for the reproductive period. The increased length of daylight 
in late winter in the middle and higher latitudes evidently stimulates 


the pituitary to release gonadotropins in sufficient amount to bring 
the bird into full breeding condition. These gon ado tropic hormones 
act upon the interstitial cells of the gonads, which elaborate the respec- 
tive hormones, testosterone (male) and esterone or estrogen (female). 
Following the breeding season (usually during the shortening of 
day length), a marked regression takes place in the gonads, along with 
all the dependent reproductive activities and conditions. It may be 
that the hormone prolactin plays a part in blocking the action or re- 
lease of gonadotropic hormones of the pituitary so that light no longer 
influences the reproductive cycle. This condition (sometimes called 
refractory period) wears off or otherwise terminates by the following 
spring. In the fall, sporadic activity similar to the breeding season 
(fall remtdesence) may occur in some species. The whole subject of 
breeding rhythm, light, and endocrines, however, is one of intensive 
study and some conflicting conclusions. 

Fertilization. The spermatozoa pass into the cloaca from which 
they are transferred to that of the female by direct contact during 
copulation (page 356). The spermatozoa propel themselves up the 
oviduct from the cloaca by means of their motile tails. The length of 
time they take is not entirely known, though it may be relatively 
short. The duration of viability in the oviduct varies; it may be as 
much as two months in the Gentoo Penguin (page 356). Fertilization 
usually takes place at the upper end of the oviduct.* 


Initial Growth. The ovum enlarges rapidly as a result of accumu- 
lations, largely protein and fatty acids, from the blood stream. It may 
increase by as much as twenty-five times the original volume in the 
first 24 hours. It passes out of the ovary into the body cavity where 
it is picked up by the adjacent funnel of the oviduct. The albumen 
and shell are laid down in the oviduct, though the yolk and vitelline 
membrane are laid down in the ovary. The yolk, albumen, and shell 
comprise about 53, 35, and 12 per cent, respectively, of the egg in 
precocial birds and 73, 20, and 7 per cent in altricial birds (Romanoff 
and Romanoff, 1949). 

The cells of the embryo begin to divide shortly after fertilization 
and continue to do so in the warm environment as the egg travels 
down the oviduct. Albumen is added by glands, followed by a thin 
membrane, and finally the hard, limey shell. Mechanical passage of the 

* No cases of parthenogenesis are known in nature, but experimental incubation of 
unfertilized eggs of the Domestic Turkey showed development of embryos in 14.1 to 
22.4 per cent (Science, 1954," 120:545-546). 



egg mass down the oviduct seems sufficient to stimulate the glands into 
activity, although chemical stimulators may be present. 

Egg Shape. The shape of the bird egg varies from nearly spheri- 
cal to relatively cylindrical (Fig. 6-4). But the commonest shape is 

Fig. 6-4. The shape of bird eggs varies from nearly spherical to sharply 
acute and the color from unmarked white to richly colored and marked. 
Top: Horned Owl, Killdeer, and King Rail Bottom: (upper row) 
Bronzed Grackle, Blue Jay, Baltimore Oriole; (lower row) Brown 
Thrasher, Ash-throated Flycatcher, Orchard Oriole. 



Fig. 6 5. Diagrams showing the closer fit of acute eggs (left) than long 

(center) or elliptical (right) ones. 


of an ovoid type in which one end is large and the other pointed. Bird 
eggs are rather varied in shape, actually; eggs of the Grebe, for exam- 
ple, are pointed on both ends, and the eggs of irtany Shorebirds and 
cliff nesters have the pointed end acute. The latter shape may have 
some survival value in cliff nesters as the egg rolls in a short circle, 
if disturbed, instead of off the ledge. The acute (or pie-shaped) eggs 
of Shorebirds and other small birds which lay a disproportionately 
large egg fit more closely together and permit the small body to cover 
a greater mass of eggs than would otherwise be the case (Fig. 6-5). 

Egg Passage. Two sets of muscles in the oviducal walls cause 
the egg to progress. The outer, longitudinal ones shorten the oviduct 
by contraction; the inner, circular muscles reduce the bore. Coordi- 
nation between the two sets of muscles and variations in tension ahead 
and behind the egg mass move it forward and shape it. Eggs with large 
amounts of albumen and yolk are likely to be elongated, while those 
with small amounts will be rounded. In the Domestic Hen, and prob- 
ably other birds, the egg is reported to move pointed end first (Ro- 
manoff and Romanoff, 1949). 

Egg Surface. The surface texture of the egg shell results from the 
molding effect of the oviduct lining. Rough linings give pitted sur- 

Fig. 6-6. The egg of the Ostrich compared to that of the Ruby- 
throated Hummingbird. The surface of many Ostrich eggs bears heavy 


faces and smooth linings, smooth surfaces. The eggs of Woodpeckers 
are smooth and glossy; those of the Tinamou are highly polished and 
porcelain-like. Heavy pitting occurs in the shells of many Ostrich 
eggs (Fig. 6-6) and lesser pitting in those of some Grebes. 

Egg Color. Glands in the wall of the oviduct secrete color in the 
form of pigment drops, especially in the lower end of the oviduct. 
Uniform color may result from color deposited in the shell material 
itself or in the last thin layer to be applied. If examined carefully, thp 
egg shell may appear layered, sometimes in layers of different color. 
Different glands in different parts of the oviduct lay down the several 
layers. Usually only the final, outside layer of shell carries the color, 
but inner layers may sometimes be colored. 

The egg of the Ani (Crotophaga) appears white on account of a 
chalky outer layer that can be rubbed or scraped off easily to reveal a 
greenish or bluish undercolor. Clearly, the final layer put on is white. 
In some birds the outer color is blue, in others it is green. But the 
commonest color seems to be light shades of brown, such as pale buff 
or cream. Blue eggs are perhaps next most common and green colors 
are rather uncommon. Other colors have been recorded at times. The 
Loon egg is a dark brown; the egg of the Emu is so dark a gray as to 
appear black. No red eggs are known, though some rich brown ones 
have been reported. It is said that the least protectively colored eggs 
are the most palatable to mammals (Cott, 1952). 

Markings. The pattern or shape of markings on shells not uni- 
formly colored vary with the species and individuals or even between 
eggs of the same bird, although all eggs of the same bird tend to be 
similar (Fig. 6-4). The markings usually are brownish or red-brown, 
secreted through the openings in the oviduct. If the pigment sets 
rapidly, the markings will be spotted or slightly linear. But if the 
pigment sets slowly or its secretion is prolonged, the markings will 
form streaks. The markings may be smeared or even form blotches 
if the egg moves much. If the egg rotates in the oviduct, the result 
will be spiral markings. 

Some aberrant changes in marking, such as streaks overlaid with 
a plain color, can be attributed to unusual happenings in the oviduct. 
Among these appears to be a reversal of peristalsis in the oviduct to 
cause the egg to reverse direction at some stage. This reversal may 
also cause the reversed egg to be enclosed within another shell, as 
well as other reported abnormalities. 

Egg Color and Life Habits. Birds that nest in holes, like Owls and 
Woodpeckers, lay white eggs. Doves also lay white eggs, as does the 
Whip-poor-will. Although the white egg of hole-nesting birds has 


popularly been attributed to the lack of need for protective colora- 
tion in the security of a cavity nest, a better explanation perhaps is 
that the white color enables the incubating bird to discern the eggs 
better in the dim light of the cavity. The fact that Whip-poor-will 
eggs are white and that the bird feeds only after dark may involve the 
same explanation. Eggs of the Nighthawk, on the other hand, are . 
streaked. Nighthawks feed by day or at twilight and do not need the 
greater visibility occasioned by white eggs. The eggs of a Nighthawk 
are exposed more in daylight also. 

The use of the markings seems associated with protective colora- 
tion. In some birds, particularly the Shorebirds and the Nighthawk, 
the eggs so closely resemble the background as to be scarcely visible, 
even to an observer at close range. The eggs of the Killdeer blend 
into the background of pebbles which the bird gathers together at the 
nest. It suggests that the gathering of pebbles marks an instinctive act 
associated with the egg pattern to increase possibilities for survival. 

Birds .that lay eggs covered by streaks usually build nests lined 
with dry grass or other vegetative material which carries out the 
linear appearance of the egg pattern. Birds that use mud, feathers, 
sticks, or leaves for the nest lining tend to lay plain eggs or eggs hav- 
ing blotches of color rather than streaks. But there are many excep- 
tions to any general rule, some of which can be accounted for on the 
basis of our knowledge of bird life but others cannot. Some may 
represent inborn traits reflecting habits of an earlier period in the 
history of the species. Thus, most of the hole-nesting birds lay plain, 
white eggs, but members of the Wren family lay speckled eggs in the 
hole nest. The Wrens probably took up hole nesting only recently, 
phylogenctically speaking, and have not yet lost the tendency for 
color pattern in the eggs. The Bluebird still constructs a thrushlike 
nest in a hole where it lays blue eggs. It is presumed to have become 
a hole-nestcr in recent Bluebird history. I lole-nesters of presumably 
long standing do not build a nest in the cavity but lay eggs on the cav- 
ity floor. Construction of a nest in a hole (really a rather unnecessary 
use of energy) is suggested as evidence testifying to the recency of 
the hole-nesting habit on the part of a bird whose construction in- 
stincts are still those of a bird which does not nest in holes. 

In general, the external appearance of the bird egg varies because 
pf hereditary, physiological, anatomical, and environmental factors. 
The fact that the eggs of a species tend to be similar for all individuals 
testifies to the overpowering influence of heredity. Even the variations 
in the eggs of an individual bird are likely to be about the same for 
all its eggs, those of the same set and season as well as those of later 
years of reproduction. 


Egg Size. The size of the oviduct determines the diameter of the 
egg, which varies in general with the size of the bird. Obviously, the 
larger the bird, the larger should be the expected egg and embryo, 
but larger birds lay relatively smaller eggs than do smaller birds. Eggs 
have been reported to increase in size with the 0.73 power of the 
body weight. Many exceptions occur in nature, and some groups 
have peculiarities all their own. Precocial birds need more food for 
embryonic development and a larger reserve at hatching, which the 
albumen and yolk provide, respectively. The size of the Shorebifd 
egg is particularly noteworthy for its large size in proportion to the 
bird body. It may be as much as ten times the size of eggs of similar- 
size Passerines (Fig. 6-7). The Ostrich has the largest egg of all liv- 

O o 

Fig. 6 7. The egg of the Shorebird on the left is ?mich larger than that 
of the Passerine bird of the same 'weight on the right. 

ing birds and the Hummingbird the smallest (Fig. 6-6). The egg of 
the extinct Elephant Bird (Aepyorms) of Madagascar reached a 
record, 2 -gallon size. 

The long axis varies more than the short axis in eggs for the evident 
mechanical reason that the distensibility of the oviduct is limited. The 
ratio of diameter to length increases in migratory birds as compared 
to semimigratory or resident ones (Averill, 1923), a variation accom- 
plished sometimes by reduction in the bore of the oviduct (in line 
with general reduction of visceral parts in migratory birds or birds of 
powerful flight) and sometimes by increase in egg volume. 

Egg Weight. The weight of the egg in proportion to the size of 
the bird declines as the body weight increases, though this may not be 
the case for many of the Fringillidae (Amadon, 1943). The egg 
weight may exceed 10 per cent of the body weight in small birds but 
be less than 2 per cent in large ones. The egg of the Kiwi weighs 
400 grams or more, which is about a sixth to a fourth of the body 



weight of 2,500 grams and holds the record for relative size (Fig. 
6-8). The birds parasitized by the Old World Cuckoo are smaller 
than the Cuckoo, but the latter lays an egg about the size of the host 
egg, a characteristic assumed to reflect an adaptation to laying in the 
nest of small birds (Chapter 13). In this case, the egg of the Cuckoo 
is about 3 per cent of the body weight instead of 10 per cent as in the 

Fig. 6-8. The Kiwi lays the largest egg for the body size of all birds; 
it may be about a sixth or more of the body weight. (Courtesy of Chicago 
Natural History Museum.) 

host. Though there are variations in size of eggs within the species 
and among eggs laid by the same bird, the eggs of each species and of 
each individual have a general uniformity. First-year females usually 
lay smaller eggs than more mature ones. (But the size of the egg de- 
clines during "old age" in Domestic Chickens.) The larger eggs of 
mature birds perhaps result from stretching or relaxing of the oviduct. 
Because bird eggs have a specific gravity of but slightly more than 
water, the weight in grams gives a rough figure also of the size in cubic 
centimeters. The volume has also been stated to be about one-half the 
product of the length times the squared diameter, although the shape 
of the egg will materially influence this, 





g 1500 






15 20 25 




45 50 55 60 65 70 75 80 85 90 


Fig. 6 9. Weight of newly laid House Wren eggs correlated with the 
average temperature for three days preceding laying. The number of egg 
weights is indicated for the respective points. (After S. Charles Kendeigh, 
"Length of Day and Energy Requirements for Gonad Development and 
Egg-Laying in Birds," Ecology, 22(1941):24f.) 

Table 6-1 
Some Eggs of Like Incubation Periods 


State at 

Egg Weight 
in Grams 

Incubation Period 
in Days 

. . . Precociai 



. . . Altricial 




. . . Precociai 




. . . Altricial 



. . . Precociai 



. . . Precociai 



Stock Dove . . . . 

. . . Altricial 



Broad-tailed Hummingbird . . 

. . . Altricial 
. . . Altricial 




. . . Altricial 



Source: Margaret Morse Nice, "Problems of Incubation Periods in North Ameri- 
can Birds," Condor 56(1954): 173-197. 


The average weight of eggs and the number per clutch may decline 
as the season advances. High air temperature may have an unfavor- 
able influence upon the size of eggs as well as on the number laid 
(Fig. 6-9). Table 6*1 shows a few examples of the size range of bird 
eggs and incubation periods for a few species. 

Number of Eggs. The number of eggs laid by a bird varies with 
the species, as has been said earlier. In general, it varies within fixed 
limits, but in some birds the number is definite. The Passenger Pigeon 
lays but a single egg, and the Band-tailed Pigeon also lays one. Other 
birds that lay but a single egg are the California Murre, Rhinoceros 
Auklet, Tufted Puffin, Petrels, and some of the Penguins. The Cali- 
fornia Condor not only lays a single egg but is believed to lay every 
other year or perhaps at even longer intervals. 

The number of eggs that a bird may be capable of laying seems to 
exceed the actual number laid in most species, probably as an adjust- 
ment for potential egg losses (Chapter 14). The Ruffed Grouse lays 
at most from 9 to 15 eggs in a set and the Bob- white 12 to 20, but 
removal of eggs so as to leave but one or two has resulted in a Grouse 
laying 36 and a Bob- white 128 before stopping. A Yellow-shafted 
Flicker similarly treated laid 71 eggs in 72 days. Other large numbers 
laid have been: Wryneck 48, House Sparrow 51, Mallard Duck 146 
(in 158 days). The Jungle Fowl normally lays a set of 11 to 14, but 
one once laid 361 and another 309 before stopping. The Turkey has 
laid as many as 100, and a caged Canary 60. The reverse experiment, 
that of adding eggs to the nest, may have the opposite effect. If a bird 
has laid but one or two eggs in a nest, artificial addition of enough to 
complete the set may result in the bird's laying fewer eggs. Birds af- 
fected by adding or subtracting eggs in the nest have an indeterini- 
ncmt type of laying; those not affected have a determinant type. Ap- 
plication of stringent standards indicates that perhaps few birds are 
really indeterminant layers (Davis, 1955). 

The amount of mineral and nutrient material in the body is limited, 
so that a bird cannot lay indefinitely without replenishing its supply. 
In the case of the Domestic Hen, experiments indicate that not more 
than an 11 -day supply of calcium can be stored in the body. The 
phosphorus supply has been enough to last 107 days on a phosphate- 
free diet. 

, The greatest egg-laying effort for the Domestic Hen (admittedly 
not a good example for comparison) seems to Be a record of 1,515 
eggs laid in eight years, but the record will probably fall soon. It is 
likely that birds have sufficient body capacity to lay all eggs necessary 
for normal production to maintain the respective species under ordi- 
nary conditions to be expected in the wild. 



Laying Time. Ovulation in most small birds seems to occur at inter- 
vals of about 24 hours. It may recur within a few minutes after an 
egg has been laid. The time needed for the actual process of laying an 
egg varies from a matter of seconds sometimes in socially parasitic 
birds like the Cowbird (pages 235, 369) to perhaps 1 to 2 hours in 
Turkeys and Geese. The Bob-white has been timed and needs but 3 
to 10 minutes at the nest to deposit an egg. Most small birds may 
require about the same length of time. 


Early Embryonic Development. Embryological development long 
has been studied in the abundant supply of developing Chicken eggs, 
so that the embryology of the Chicken, especially in the early stages, 
is well-known, standard zoological demonstration material (Figs. 

Development begins even at the moment of fertilization and con- 
tinues in the egg so long as conditions are favorable. Rapid develop- 
ment occurs as the egg passes down the oviduct where temperature 
is optimum. Because an egg takes several hours to pass (it varies from 
a few hours to twenty-four or perhaps more), the embryo may be 


Fig. 6-10. The chick embryo during its first 72 hours of development. 



Fig. 6 1 1. Three stages in development of the chick and the embryonic 


several hours developed by laying time. In the Chicken egg, the 
embryo reaches the primitive-streak stage when laid. Fig. 6-12 shows 
the structure of a fertile egg. 

Incubation Rate. The incubation rate varies with the heat sup- 
plied by the incubating bird. It slows up (or even ceases) when the 
bird leaves the nest (page 371) and resumes after heat again reaches 
the embryo. It cannot be shortened by exposure to high temperature. 
Development ceases altogether below about 80 F. (21 C). Yet the 
cooling periods attending the departure of the incubating bird actually 
may stimulate development of the embryo and thereby compensate 
for lowered temperatures (Kendeigh, 1940). Incubation may be sus- 
pended for some time without injury to the embryo. In the case of a 
newly laid egg, because embryonic development started at fertiliza- 
tion, this suspension may be for 3 weeks or even more in Bob-white 
and other birds laying large numbers of eggs. It may be for only a few 
days in the average Passerine bird or not at all in birds like the Ameri- 
can Cuckoos that start incubation with the first egg. (In the last- 
named case, the first-laid egg hatches several days before the last one; 
the size of the young varies correspondingly.) The body temperature 
of the embryo and nestling fluctuates with incubation and brooding 
by the parents. 

Air temperature during the suspension period may adversely affect 
the hatchability of the newly laid eggs. Birds native to warm regions 
lay eggs that can stand high air temperatures (some of which may 
reach the nineties at mid-day) and still hatch well. But eggs of more 
northern species have shown a much lower hatching rate when simi- 
larly exposed; young hatched from eggs thus exposed to high tem- 
peratures are likely to be of lowered vigor. The same principle seems 
to hold also for northern species that lay eggs so early as to be subject 
to snow, as actually happens to eggs of Horned Larks, Horned Owls, 
and other early nesters. The eggs of the Penguins at times are sub- 
jected to cold without destroying their hatchability. It is likely that 
the effect of temperatures between laying and incubation influences 
both the southern limits of northern species and the northern limits of 
southern species. 

The incubation temperature approaches that of the incubating bird, 
which ranges from about 100 F. to 110 F. in different species. (It 
has been reported that the most rapid development occurs at a tem- 
perature several degrees above the body temperature for the species.) 
The normal bird has a temperature rise in the day and a drop at night, 
but this variation is not so well marked in the incubating bird that re- 
mains relatively motionless. (It is said that for the reason of this 
motionless condition, the incubating, ground-nesting bird gives off 


little scent.) Actual measurements show wide variations of egg tem- 
perature in various parts of the nest. Perhaps the turning of eggs by 
the incubating bird shifts them about and thereby equalizes develop- 

The temperature of the egg varies also with construction of the nest. 
The more substantial and better insulated nests of boreal birds retain 
heat better than the thinner nests of birds living in a warmer climate. 
The type of nest construction by a bird may give some indication 
of the ancestral home of the species. Species invading a warm region* 
from a cold one still construct relatively more solid nests, as their an- 
cestors no doubt did and as their boreal relatives still do. The Cor- 
vidae, for example, all build thick nests (Fig. 18*7), even those in the 
Sonoran Life Zone. 

The incubation period of precocial birds lasts longer than that of 
altricial ones largely because development is carried to a more ad- 
vanced stage. But when developmental stages are alike, larger eggs 
tend to require longer incubation periods than do smaller ones. Long 
incubation periods relative to egg size occur among the Casuariiformes, 
Apterygiformes, Sphcnisciformes, Proccllariiformes, Falconiformes, 
Psittaciformes, Strigiformes, and Trochilidae. Short incubation pe- 
riods characterize Struthioniformes, Rheiformcs, some Anatinae, 
Turnicidae, Columbidae, Picidae, and most Passeriformes. Knowledge 
of incubation periods, however, is almost too confused for any con- 
clusions (Nice, 1954). 

Development of the Germ Layers. The bird egg is strongly 
polylecithal (which means that it has a large amount of yolk) and 
telolecithal (which means that the yolk is concentrated on one side). 
The chalaza suspends the yolk in the albumen and permits it to rotate 
so that the embryo rests on the top side (Figs. 6-10, 6*12). Because 
the bird egg has much yolk, cleavage involves only a small disc lying 

sheik ^^^==^ == ========^^ ^vitelline membrane 



^'yellow yolk" 

Fig. 6 1 2. Diagram of saggital section of a Chicken egg to show 



atop the yolk. This is sometimes described as meroblastic discoidal 
cleavage. Three layers, ectoderm, endoderm, and mesoderm, give rise 
to all parts of the body (Fig. 6-11). 

Development Rate. After about 27 hours of incubation in the 
domestic chick, the head begins to take shape at the forward end of 
the primitive streak and soon the future parts of the brain become 
distinguishable. By 30 hours the heart is recognizable, and blood ves- 
sels soon appear. Blood begins to circulate toward the end of the, 
second day of incubation. Organs for securing food and oxygen and 
for eliminating waste products follow soon after. These essential 
acts are carried out by the yolk sac and allantois (Fig. 6-12). The 
shell and egg membranes are porous so that carbon dioxide can pass 
out and oxygen in with considerable freedom. During the fourth 
day, limb buds appear as the forerunner of the future legs and wings. 
By the fifth day most of the parts of the body have been established. 
In the Domestic Chicken, the embryo is about half developed by the 
tenth day and is completely developed by about the twenty-first day. 

Although we may speak of birds as altricial or precocial (Fig. 
6*13), depending upon their stage of development at hatching, con- 
siderable variation occurs within these groups. It is logical that some 
intermediate stages between precocial and altricial conditions should 
occur (page 376). The important matter of comparative embryonic 
stages of development for different wild species has not received much 
attention from embryologists. 

Birds of higher body temperature have slightly more rapid em- 
bryonic development than others. In addition, we may expect that 
embryos of some species might very well grow faster than those of 
others. But the large egg of the meat breeds of the Domestic Chicken 
(3.5 cubic inches) still takes the same 21 days as the ancestral Jungle 
Fowl with its smaller egg (1.6 cubic inches). The incubation time 
thus seems to be a fixed part of the hereditary complex of the species. 


PATTON, BRADLEY M., Early Embryology of the Chick. Philadelphia: The Blakiston 
Co., 1952. 

PRESION, T. W., "The Shapes of Birds' Eggs," Auk, 70(1953): 160-182. 

John Wiley & Sons, Inc., 1949. 

SHUMWAY, WALDO, Introduction to Vertebrate Embryology. New York: John 

Wiley & Sons, Inc., 1948. 
*STORER, TRACY I., General Zoology. New York: McGraw-Hill Book Co., Inc., 1951. 

WEIMAN, H. L., An Introduction to Vertebrate Embryology. New York: McGraw- 
Hill Book Co., Inc., 1949. 

WEICHERT, CHARLES K., Anatomy of the Chor dates. New York: McGraw-Hill Book 
Co., Inc., 1951. 



It may be redundant to say that unless a creature has feathers, it is 
not a bird. Yet the possession of feathers does characterize a bird; no 
other animal has the slightest trace of them. Their origin and evolu- 
tion belong to that obscurity characteristic both of soft parts and of 
long-ago. Hardly an adequate theory has been offered for their ori- 
gin, mute testimony to the remoteness of their beginning and to our 
consequent lack of knowledge. The theory that the feather originated 
from a scale in essence suggests that the scale became lengthened and 
thinned down, its edge frayed out and split, the upper surface to be- 
come the feather proper and the lower one the aftershaft. Another 
theory suggests the mechanism more than the origin. It proposes that 
the feather developed originally as an outgrowth from the lower 
layers of the skin and pushed out through the junction between scales. 


Tracts. Although feathers may grow more or less uniformly over 
the body of some birds, as in the Penguins, they generally grow in 
definite places known as feather tracts. The term pteryla has been ap- 
plied to a tract and apteria to the intervening bare spaces. The terms 
ptilosis, pterylosis, and pterylography have been variously applied to 
feather arrangement. There are eight chief tracts (Figs. 7 1, 7 -2). 

Head or capital 

Spinal or dorsal 

Humeral (located on upper arm) 

Femoral (located on thigh and running to anus) 

Ventral (from chin to anus, dividing along each side of belly) 

Caudal (tail feathers and coverts) 

Wing or alar (all wing feathers outside the humeral tract) 

Leg or crural (all leg feathers except those in the femoral tract ^ 













5 CM. 








Fig. 7-1. Dorsal view of pterylography of a Passerine bird (Aphe- 
locoma coerulescens insularis, Scrub Jay). (After Frank A. Pitelka, 
"Pterylography, Molt, and Age Determination of American Jays of the 
Genus Aphelocoma," Condor, 41(1 945 '):229-260.) 

Kinds of Feathers. Four regular types of feathers occur, with an 
additional fifth kind, the powder-down feathers found in some water 
and land birds. ( 1 ) Contour feathers, as the name implies, are the out- 
side feathers, the ones that give the bird its shape. (Adult feathers 
may be called teleoptiles to distinguish them from those of the 
young.) (2) Down feathers cover the young bird (in which case they 
are termed neossoptiles) ; they may also be found in the apterias and 
tracts of adults (in which case they are termed plwnmles) . Scattered 
through the feather tracts are (3) sevriplumes, similar to contour 
feathers but without the interlocking construction of the latter. At 
the base of the contour feathers are hairlike feathers called (4) filo- 
plumes (Chandler, 1916). 

Special feathers called powder-down have an extraordinary func- 
tion, at least in the Bittern. They are a most effective "dry shampoo" 


























5 CM. 


Fig. 7-2. Lateral view (a) and ventral view (b) of the pterylography 
0f a Passerine bird. (After Frank A. Pitelka, "Pterylography, Alolt, and 
Age Determination of American Jays of the Genus Aphelocoma," Condor, 



for cleaning the plumage, even of such soil as eel slime (Percy, 1951). 
Other birds have powder-down (e.g., Herons, some Hawks, Wood 
Swallows). Many birds, especially water birds, use secretions from 
the oil gland (uropygial gland) for dressing the feathers. Without 
these secretions, feathers of Waterfowl lose their wet-proof quality 
and become bedraggled sooner than usual. 

Feather Growth. The feather grows from a feather follicle, rec- 
ognizable in the Chicken embryo at the 144-hour stage when a papilla 
projects from the skin. The bird skin itself is thin (Fig. 7*3). The 

corneal layer 

1^ germinative 


Fig. 7*3. The bird skin. The integument is characterized by thin 
epidermis and derwis covering a thicker subcutaneous layer where fat 
may be stored. 

dermal layer of the projecting papilla becomes cornified to form the 
feather, while the mesoderm supplies nutrients only (Fig. 7*4). The 
feather succeeding the natal down and all subsequent feathers grow 
by periodic stimulation from the same follicle. The natal down itself 
is pushed out on the tip of suceeding feathers. But all subsequent 
feathers grow only after the predecessor departs, which may be by 
a regular molt or by the irregular loss of the feather. The loss of a 
feather outside the molting season is enough to stimulate the follicle 
to replace that feather, but the replaced feather will represent the 
plumage of the succeeding molt. 

The cross-barring and growth marks on some feathers indicate 
daily growth believed associated with drop in blood pressure and 
increases of rnitotic activity at night when the bird is at rest. 





feather pulp 


annular groove 

peridium N 

feather shaft 
feather barbs 


feather follicle 

Fig. 7-4. Development of a contour feather. (Top) Early stages as 
seen in cross-section; (bottom) view of later stage. 

Structure. The feather is indeed a marvel of nature (Fig. 7*5). 
Trie quill or calamus is the part attached to the bird. It is really the 
body end of the shaft-, the other end, to which the vane or *web is 




Fig. 7-5. (Left) A contour feather to show the main parts. The inner 
and outer vanes together form the web. (Middle) Three types of feathers. 
(Right) Two adjacent barbs magnified to show the barbules and booklets 
that interlock to form a flexible but tight webbing. 


attached, being the rachis. The web of the feather consists of proc- 
esses known as barbs, each one bearing further branches called bar- 
buleSj which in turn bear smaller branches called barbicels. These 
barbicels are booklets (haimili) that interlock with adjoining ones to 
give the feather its webbed effect (Chandler, 1916). The booklets 
are so constructed that if they become separated, a little manipulating 
by the bird's head or bill will put them back together. Preening of 
the feathers is in part restoring of the hooked condition. The number 
of barbs on a feather may run into the hundreds, barbules into the 
thousands, and booklets into the millions. 

Under-feathers used for insulation have a fluffier character owing 
largely to the absence of the booklets which hold the barbs together. 
The wing feathers of the flightless Ostrich have no booklets, which 
gives them a fluffy, curly character prized by fashion designers. 
Egret plumes likewise have no booklets. 

Attached near the base of the quill on the inner side of body 
feathers appears in some birds a small feather called the aftersbaft. 
In some this may be only a thread, in others a diminutive feather, and 
in a very few others (e.g., Emu and Cassowary) it may be as large 
as the feather itself. 

Number of Feathers. The number of feathers on the body varies 
with the species and possibly with the season. The number seems to 
vary also with the systematic position of the species. The greatest 
numbers are present immediately after molt and decline as feathers 
are lost. In general, larger birds have greater numbers of feathers and 
water birds have more than land birds. A few actual counts arc listed 
in Table 7-1. (The first four are from Wetmorc, 1936.) 

Table 7- 1 
Actual Counts of Bird Feathers 



Body Feather 
weight weight 
(grams) (grams) 

Mourning Dove 


152.7 11.7 

2.8 .2 
41.4 1.9 

(1,246 on head, 3,051 
on body) 

(20,177 on head and 

American Robin 

2 973 

Ruby-throated Hummingbird .... 


Cowbird (female) 



. . . 3,235 

Cowbird (male) 


Glaucous Gull 


Domestic Chicken 


Mallard Duck 


Whistline Swan . .' 


neck, 5,039 on body) 



Number and Sequence. The molt of birds follows well-devel- 
oped patterns in the various species; some molt once, some twice, and 
some even three times a year. Molt usually occurs in fall and spring, 
especially among migratory birds; it may be influenced by day length 
(Lesher and Kcndeigh, 1941). The fall molt thus follows the wear 
and tear of the breeding season and starts the bird off for migration 
and winter in a new suit of clothes. In the same way, a spring molt 



Fig. 7-6. The Ptarmigan (Lagopus) 'wears a mottled grayish or brown- 
ish plumage in swinncr but molts to a 'white one for winter. 

starts the bird out in a new suit for the coming breeding season. The 
Ptarmigan with its three molts a year changes from the white winter 
plumage to the brown and white breeding plumage late in winter; 
during nesting it molts to a gray summer plumage; and in the fall, it 
goes again to the winter white (Fig. 7-6). (There is one race of 
Ptarmigan, the Scotch Grouse, which does not don the white plum- 

Molts usually begin at one side of an area (as in the primaries) on 
both sides of the body. In the molt of the primaries, for example, the 
respectively positioned feathers on both wings drop more or less to- 
gether (which maintains flight balance), but the sequence may vary 
somewhat (Fig. 7-7). Gallinaceous birds start the wing molt at the 
bend and shed the primaries outward. Ducks and Geese, on the other 
hand, shed all primaries at once, so that for a few days or at most a few 
weeks, the birds are flightless. In the case of young birds, the sequence 
of acquiring feathers and the progress of molt can be used as an indi- 
cator of age and hatching date. The sequence of tail molt may be 



Fig. 7-7. Dorsal surface of left iving of the Sharp-shinned Haivk to 
show molt of remiges and greater coverts. Secondaries are numbered. 
Shaded quills represent new feathers, white quills and feathers, and short 
quills those that are growing. (After Alden H. Miller, "The Significance 
of Molt Centers Among the Secondary Remiges in the Falcomformes" 
Condor, 43(1941):! 13-1 If.) 

constant or somewhat variable (Fig. 7-8). The rate of growth may 
become less as the feather grows longer (Figs. 7-9, 7*10). For the 
Cardinal, however, growth of tail feathers has been recorded as 0.08 
inches for sixteen days, 0.11 for the next seven days, and 0.12 for the 
next four. The average daily growth for the Carolina Chickadee has 
been reported as 0.05 inches and for the Red-eyed Towhee, 0.08. 

Altricial birds generally hatch in a near-naked condition (Fig. 
6-13), about the only feathers being a few tufts on the head and in 

Fig. 7*8. Replacement dates of tail feathers in a young -female Golden 
Eagle (hatched April 19, 1943). (After Malcolm Jollie, "Plumage Changes 
in the Golden Eagle," Auk, 64(1941):549-516.) 

the feather tracts (page 376). Most or all Woodpecker young are 
naked.) In precocial birds like the Domestic Chicken the body is 
clothed in natal down. The molt by which down is replaced by the 
juvenile (sometimes called Juvenal) plumage is termed postnatal molt. 





10 20 30 40 50 60 70 80 90 100 110 

Fig. 7-9. The feather grows more slowly as it lengthens (Red-shafted 
Flicker). Circles and solid line are primary feathers, crosses and dashed 
line are secondaries. The numerals indicate number of data for each cal- 
culation. (After Frederick H. Test, "Molt in Flight Feathers" Condor, 

o 100 

< 80 



* 60 


S 40 




Fig. 7*10. The percentage of final growth attained by nestling Long- 
billed Marsh Wren (shaded bars) and House Wren (unshaded bars) by 
the twelfth day of growth. (After Wilfred A. Welter, ^Feather Arrange- 
ment, Development, and Molt of the Long-billed Marsh Wren," Wilson 
Bulletin, 48 ( 1 936 ):25 6-269.) 

The juvenile plumage in altricial birds develops in the nest so that 
they are well attired before departing. Precocial birds depart from 
the nest when they still have natal down. The juvenile plumage is 
lost at the postjuvenile (sometimes called postjuvenal) molt, by which 


the first winter plumage is obtained. This plumage may differ from 
that of the adult or it may be similar. The first winter plumage re- 
mains all winter, after which the prebreeding (prenuptial) molt of 
late winter or early spring changes the plumage from the first winter 
to the breeding (nuptial) plumage. In some species, the young birds 
during their first breeding season retain an immature type of plumage, 
which is designated as the first breeding plumage to distinguish it 
from the fully adult type. This may occur in males of the American 
Redstart and Red Crossbill. 

The breeding plumage is shed during the post breeding (postnup- 
tial) molt. In cases where the birds require several years to attain the 
full breeding plumage, this would be called first postbreeding molt. 
The plumage donned is called the winter plumage. It usually is the 
same as the plumage worn the previous winter by birds of the previ- 
ous year's age class. If it differs from that of the first winter, it would 
be termed the second winter plumage. This plumage is lost at the 
second prebreeding molt. Only in a few cases (a few Eagles, Gulls, 
and others) does the difference in plumage continue. If it does, the 
plumages would be called second, third, or fourth breeding plumages, 
and third or fourth winter plumages; the molts would have parallel 

Eclipse Plumage. Among a few ducks (perhaps more than is 
generally realized), the male changes at the postbreeding molt into 
a plumage resembling that of the female. In a short time, usually only 
a few weeks, the bird molts again into the characteristic male plumage. 
This transitory summer plumage is called the eclipse plumage. Its ap- 
pearance has long been known for the spectacularly colored Mallard 
and Wood Ducks. The eclipse plumage is said to represent the win- 
ter plumage worn for a short time and then to be lost by a shift of 
the prebreeding molt from the end of the succeeding winter forward 
several months to appear in summer. The plumage worn most of the 
year may really be the breeding plumage (Pettingill, 1956). 

Moltless Change. Though changes in plumage ordinarily occur 
at molts only, a few birds have special means by which a plumage 
color or pattern change occurs without molt (though the feathers 
themselves change only at molt) which is termed aptosochromatism 
(sometimes aptosochroism). Several birds acquire the breeding plum- 
age by wear or breaking of feather tips. The male of the common 
House Sparrow wears a long black "bib" in the breeding season. But 
in the winter season, this black area is much restricted because of 
gray feather tips which hide it. These wear or break off during the 
late winter and the full black front appears by the breeding season 



Fig. 7*11. The change in black from the narrow "/;//>" of winter (a) 
in the Home Sparrow to the breeding plumage (b) takes place by the 
breaking off of the gray tips (c) to reveal the black underneath. 

(Fig. 7-11). In the same way, the male Bobolink dons his handsome 
black and white breeding plumage by loss of yellowish tips hiding 
the black. A brownish wash over the wintering Snow Bunting hides 
the fact that the feathers themselves are largely white. In spring the 
brown tips fall away to reveal the clear white of the breeding 

In all cases, however, one must bear in mind that any color is likely 
to fade. The clear browns, reds, or blues may fade greatly as the 
season progresses, so that they may have changed their appearance 
markedly by the time of molt. The green of the Cissa or Chinese Jay 
changes to blue in museums and zoos owing to the evaporation of 
volatile yellows. The deep orange-yellow plumes of the Twelve- 
wired Bird-of-paradise fade to pale lemon-yellow in museums. In 
addition, soiling and mechanical wear may materially alter the 


The Nature of Color. Feathers achieve color in two standard ways, 
pne chemical and the other structural, but our knowledge of the sub- 
ject leaves much to be desired. Some colors appear to involve both 
chemical pigments and the structural break-up of light. 

Animal pigments are sometimes called biochroities, of which three 
are reported to be common in birds: carotewoids, tetrapyrroles, and 
melanins. The carotenoids are red, yellow, and orange pigments of a 


fatty nature. They give red color to feathers, for example, as well as 
the yellow in the legs and bills of some birds. Some yellow feathers 
may carry a carotenoid pigment, while others may be yellow from 
structural color. 

Tetrapyrroles may be of hemoglobin pigment (porphyrins) or bile 
pigments. The former gives the red color to the comb and wattles 
of the Domestic Chicken. The bile pigments may give eggshells either 
the ground color or markings. Two tetrapyrrolic pigments may well 
be mentioned because of their unique character: turac'm, a red pig- 
ment (containing copper); and turacoverdm, a green pigment. Both 
are found in species of Musophagidae of Africa. 

Melanins are usually brown, and in concentration they appear 
black, as in the Raven. Both the physical and the chemical nature of 
the melanin may vary the color. 

Structural colors are reported as solid colors or as iridescent ones. 
The neck feathers of Crackles show black, but when the light strikes 
at the right angle from the observer, they appear as a metallic green 
or purple. The throat of the Hummingbird likewise shows iridescence 
in a pronounced manner. 

Small irregular cells overlying pigment granules (melanin) break 
up the light falling upon them; the broken-up light thus shows against 
the dark background of the pigment (Fig. 7*12). Such structural 
colors can be altered physically, but not by chemicals unless the 

(a) - 



Fig. 7*12. The color-producing cells in the Blue Jay feather, (a) 
Cornified layer, (b) color-producing layer, and (c) layer of pigment 
granules. (After Carl Goiver, "The Cause of Blue Color as Found in the 
Bluebird (Sialia sialis) and Blue Jay (Cyanocitta cristata)," Auk, 53 


reflecting cells are penetrated and filled by a chemical or the under- 
lying pigment is bleached. If the structure is altered physically as by 
crushing or grinding, the color disappears. Pigment colors, on the 
other hand, can be ground or crushed without changing their color. 

Abnormal Colors. Variations of normal color appear in the wild 
and others can be induced in the caged bird. Stains and soil are some- 
times called adventitious colors. The rusty color sometimes seen on 
the plumage of Swans and Snow Geese is of this nature. Wood- 
peckers develop a very dark breast from rubbing against tree trunks, 
and Crossbills may also get very dark from pitch and other soil. House 
Sparrows living in cities acquire a dark soil of soot which their 
country cousins do not. American Sparrow Falcons that feed upon 
grasshoppers in burnt stubble will get very dark. 

An absence of pigment results in white, and the condition is called 
albinism. All light rays are reflected by the "pigment holes" thus left 
in the feathers just as foam on a wavy sea looks white from the same 
type of reflection. The defect resulting in albinism may be complete 
(in which case the bird has pink eyes and soft parts) or partial, and 
involve only a patch of feathers or even a single one, which will show 
white in place of normal coloration. Temporary albinism may be 
caused by dietary or circulatory deficiencies at the time of feather 

Albinism is distinct from white coloration, as in the entire plumage 
of Swans or in the few spots on a Robin's body. Such white colora- 
tion does not involve the soft parts, and the eyes, legs, and bill have 
the usual color. Genetic studies show that among breeds of the 
Domestic Chicken, some white patterns (even the entire plumage as 
in White Leghorns) are dominant over color, the genes for the latter 
being recessive (Chapter 20). Albinism when inherited is recessive 
to the normal plumage. White plumage grown by the adult and 
young Ptarmigan at the postbreeding and post juvenile molts, re- 
spectively, is a true white one, not albinism. (It may be added that 
the white plumage will come and go at the proper seasons, snow or 
no snow.) Albinism has been reported in America for many birds, 
among them the American Crow, Raven, Bronzed Crackle, Cowbird, 
Red-winged Blackbird, House Sparrow, American Robin, Chimney 
Swift, and Barn Swallow. Albinos may be accepted with little dis- 
crimination among some species at some times, or they may be har- 
rassed by other members of the species. 

Albinism has been classified as total albinism, incomplete albinism, 
imperfect alb'mmn (dilution), and partial albinism. The last named 
may be random or specific (Nero, 1954). 


In addition to albinism, abnormal color conditions have been found 
and named melanism, erythroism, and xanthochroism. Melanism 
occurs by an excess of brown or black in the feathers. It has been 
reported in the Snipe and Skylark, and it occurs fairly often among 
some Hawks, notably the American Rough-legged Hawk and the 
Swainson Hawk. It seems established that these two species have 
melanism as a color phase. 

Erythroism, the excess of red, occurs rather uncommonly in nature. 
In the 1920's, it appeared in Bob-whites of the southeastern states, but 
practically disappeared in a few years (page 395). Breeding experi- 
ments showed that it was inherited as an apparently incomplete domi- 
nant character. Both wild and confined birds exhibited a weaker 
condition than normally colored ones (Cole, Stoddard, and Komarek, 
1949). Most mutations seem to accompany physical weakness; red 
color phases, however, are normal in some birds and the birds do not 
exhibit physical weakness. 

Xanthochroism appears in Parrots, with yellow pigment replacing 
the green. In the West, Red-shafted Flickers appear with orange, 
yellowish-orange, or even yellow where the color should be red, or 
orange-red. The Pacific Northwest reports more such birds than 
areas nearer the Yellow-shafted Flicker range. This is in accord with 
the belief that such birds are not hybrids (page 391) but possible 
cases of xanthochroism, probably through absence of the normal red. 

Dimorphism. Color phases of normal dimorphic conditions are 
termed dichromatism. The Screech Owl, for example, appears in red 
and gray color phases. Highway kills in Wisconsin showed 61.3 per 
cent to be the gray phase. Red and gray color phases also occur in 
the Pygmy Owl and Ruffed Grouse. Such color phases may be dis- 
tributed regionally, so that in some areas one phase is common and in 
others it may be rare. There is evidence to indicate that the Blue 
Goose is a regional color phase of the Snow Goose. If males and 

Fig. 7-13. The Red-winged Blackbird (a) and Hooded Warbler (b) 
illustrate sexual dichromatism. The front bird of each pair is the male. 



females differ in shape or color, it may be termed sexual dimorphism 
or dichrowatisw, respectively (Fig. 7-13). Often this involves ap- 
preciable difference in size. 


Coloration in the bird world has both visual and nonvisual values. 
The nonvisual ones themselves perform general functions irrespective 
of species, whereas a particular visual use may be the property of a 
single species only. No doubt color itself functions in both pattern 
and posture, as seen in courtship activities (Chapter 18). 

The three nonvisual values are absorption of light or heat rays, 
reflection of these waves, and the deposition of excretory products. 
It is well known that dark objects absorb more light and heat rays 
than lighter colored ones. Possibly the lighter plumages of desert birds 
serve in part for reflection of light and heat rays, along with the pro- 
tection to the bird offered by protective coloration. A bird absorbs 
radiation directly from the sun, reflected from sky and clouds, and 
reflected from earth and objects. It will radiate long infrared rays 
(Fig. 7-14). 


Fig. 7-14. A bird receives heat by radiation from the sun directly, 
reflected from sky particles and clouds, and reflected from the earth and 
objects. It will lose long infrared rays by radiation. 





The many visual values of coloration require careful and judicious 
study, which in the light of our present knowledge can hardly be at 
all complete. No doubt many colors and patterns have more than one 
value, which is somewhat the same as saying that they have more 
than one function. Colors as we see them may not be the same as 
those seen by the bird, just as the eye and photographic film do not 
"see" color the same way. They may not even be seen the same by 
the bird's enemies, friends, or prey. 

Visual values may be conveniently listed as concealing colors, 
warning colors, recognition colors, and display colors. All are asso- 
cited with pattern, posture, position, and placement (Fig. 7 15, 7 16). 

Fig. 7-16. The lighter under parts break up the solidity of the body by 
obliterating the usual shadow. 

Concealing Coloration. Concealing coloration serves to protect 
its posssessor from detection by its enemies or prey as the case may be 
(page 153). The grayer colors of the desert birds may help them to 
blend into the gray desert vegetation. The general brownish streak- 
ings of so many grass birds may conceal them in the vegetation. The 
Bittern is well known for its combining color and pattern with 
posture to effect concealment (Fig. 7-15). It will stand still with 
neck and head extended vertically. It even will turn its neck and 
head slowly as an observer moves about, always presenting the well- 
blending front. If need be, it will sway with the wind-blown vegeta- 
tion, presumably to further the deception. 

Grouse nest on the ground, and the females particularly blend with 
the ground for concealment. The Ruffed Grouse wears browns to 
match the humid forest floor, as do also the females of the Spruce 
and Franklin Grouse. The female Blue Grouse of the interior is gray 
in accord with the grayer aspect of the dry landscape. This is in 


contrast to the browner female Blue Grouse of the more humid coast. 
The Bob-white, Meadowlark, Snipe, Nighthawk, and others show 
similar concealing patterns. 

But some of the Plovers and other birds of the shore zone wear 
rather bold ruptive patterns (disruptive) that break up the body out- 
line and effectively camouflage it. This tendency for bold ruptive 
patterns seems an effective technique for many birds of the shore. It 
is used rather freely also by birds of the treetops and of many other 

Many birds are difficult to see, even in the open, because of counter 
shading. The Shorebirds have light under parts, which break up the 
solidity of the body by lightening the part the eye of an observer 
expects to appear dark from a shadow. The dark back also breaks 
up the solidity normally given by highlights, so that the bird "flattens" 
into the background (Fig. 7-16). 

Flash Colors. Many birds have brightly colored patches that may 
serve to give warning, as when a Mockingbird flashes its white wings 
as it approaches an interloper. When startled, the Meadowlark flashes 
its white tail. It does so also in conflict with other Meadowlarks. 
In some cases, the sudden flash of white or other color may confuse a 
pursuing enemy, as when a Meadowlark suddenly flashes its white 
tail in a Cooper Hawk's face before dropping into the grass. A 
Meadowlark upon alighting may flash the white tail patches and then 
quickly creep off a few feet, which act has been interpreted as an 
action (common to many birds) to cause a "positional confusion" by 
attracting attention to the place where the bird no longer can be 
found. Inasmuch as birds of prey seldom continue a chase once 
foiled, this may be an effective trait. 

Recognition Marks. Mammals use their sense of smell to de- 
termine the species and sex of another animal and also to ascertain 
whether it is friend or foe. Compared to gaily colored birds, no 
mammal has brightly colored fur, and few even have bright markings 
or striking patterns. It seems logical that bird colors in part have 
evolved so that one bird may recognize another or be recognized itself. 
Among birds of striking colors, there appear to be innate recognition 
patterns; among somber-colored species, these patterns seem to be 
associated with posture and display. The Cardinal seems to have no 
trouble in recognizing a male as such, but the male Mockingbird ap- 
parently must bristle up to every new Mockingbird before finding 
out whether it in turn bristles up like a male or acts submissive like 
a female. But a Mockingbird seems quite able to recognize its mate 
after they have paired, and this ability seems to be associated with 

PLUMAGE 1 3 1 

shape, action, carriage, voice, and many other individual attributes of 
an animal (see also Chapter 18). 

Among Yellow-shafted Flickers, the male bird wears marks under 
each chin, like mustaches (Fig. 7-17). An experimenter added an 
artificial mustache to the female of a pair and her mate promptly 
attacked her as he would an intruding male (Noble, 1936). Cock 
feathers affixed to the tail of a hen will get the same response from 
roosters. It seems certain that these male marks are recognized by 
others as denoting a male. 

Fig. 7-17. The "mustache" 011 a Flicker signals walencss to other 

The white in the tail feathers of a Junco probably serves as a 
recognition sign to keep the flock together in winter. Anyone who 
has watched a flock can detect the flash of white in the tail as a bird 
flies. The twittering of a flock no doubt is also part of the recogni- 
tion and flock-maintenance mechanism. 

Display Coloration. The large number of known examples of 
coloration associated with display leaves little room for doubt as to 
their purpose, although observers may conscientiously differ in inter- 
preting bird behavior (Fig. 7*18). Birds having conspicuous patches 
of color are prone to exhibit them in characteristic ways and at 
definite times, all of which suggest a purpose, but we must not confuse 
this with consciousness (see Chapter 18). Many of the color patches 
used in display no doubt serve as signaling devices (sewmitic colors) . 
The American Redstart's brilliant patches of red amidst the black on 
body and tail probably serve variously as ruptive patterns, recogni- 
tion marks, and display patterns. Anyone who has watched the 
Redstart fan out its tail will appreciate the apparent emphasis by this 
bird on display of the color and pattern in its plumage. 

The male Ruff of Europe has a large resplendent collar which he 
may suddenly open immediately in front of the female. Many Wood- 
peckers have display colors which they show, such as the red or yel- 
low linings of tail and wings of Flickers, displayed in courtship and 
conflict. The Greater Spotted Woodpecker displays the red under- 



tail before a rival much as a Flicker does (Fig. 7-15). The Killdeer, 
along with many other birds, feigns injury to attract an intruder from 
its nest or young. Prominent in the Killdeer act is the display of the 
rusty red upper tail coverts (Fig. 7-18). 

Fig. ? 18. Display of breast, throat, tail coverts, and crest by (a) Tit, 
(b) Hummingbird, (c) Killdeer, and (d) Kinglet. 

The display performances of the Tetraonidae have long been 
known, but many other gallinaceous birds also carry on elaborate dis- 
plays. Among the commonly known ones are those of the Turkey and 
Peacock. In the case of the Turkey, the bare skin of the head may 
become surcharged with blood and change from a steel blue to a 
bright red. The combs over the eyes of the displaying male Blue 
Grouse (commonly called "Hooter" by westerners) change in dis- 
play from a yellow to a brilliant orange-red. 


* ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 


* ALLEN, GLOVER M., Birds and Their Attributes. Boston: Marshall Jones Co., 1925. 

* ARMSTRONG, EDWARD A., Bird Display and Behavior. London: Lindsay Drummond, 

Ltd., 1947. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
BOULTON, RUDYARD, "Ptilosis of the House Wren (Troglodytes aedon aedon}? Auk, 


CHANDLER, ASA C., A Study of the Structure of Feathers, with Reference to Their 
Taxonomic Value. University of California, Publications in Zoology, 13(1916): 


COTT, HUGH B., Adaptive Coloration in Animals. New York: Oxford University 

Press, 1940. 
DWIGHT, JONATHAN, JR., "Sequence of Plumages and Molts of the Passerine Birds of 

New York," Annals of the New York Academy of Science, 13(1900):? 3-360. 
LILLIE, FRANK R., "The Physiology of Feather Pattern," Wilson Bulletin, 44(1932): 

MAYER, F., and A. H. COOK, The Chemistry of Natural Coloring Matters. New York: 

Reinhold Publishing Corp., 1943. 
MICHENER, HAROLD, and JOSEPHINE R. MICHENKR, "The Molt of House Finches of the 

Pasadena Region, California," Condor, 42 (1940) -.140-1 53. 
SUFTON, GEORGE MIKSCH, The Juvenal Plumage and Post-Juvenal Molt in Several 

Species of Michigan Sparrows. Cranbrook Institute of Science, Bulletin No. 3, 1935. 
THAYER, GERALD H., Concealing Coloration in the Animal Kingdom. New York: 

The Macmillan Co., 1909. 


Age and Sex 
in the Bird 

Our knowledge of age and sex ratios in wild birds is scanty at best, 
and this chapter assembles some available information on these im- 
portant subjects. Further material will be included in other chapters, 
such as those on bird ecology. Because the sex of a bird can often 
be told at a glance, our knowledge of sex ratios and their distribution 
may be more extensive than our knowledge of age. 

The following terms have been suggested (Wood, 1946) to desig- 
nate young birds: nestling not ready to leave the nest; fledgling 
ready to leave the nest but still under parental care; juvenile out of 
the nest but its post juvenile molt not yet completed; immature in its 
first winter plumage. 


There are five factors that must be considered in discussing age. 
The first is life expectancy, which means the time left to the individual 
at any particular age. Life expectancy differs from the average age, 
which is the average for the population or group as a whole. The 
maximum age is still a third factor, and probably represents the 
potential years that an organism might survive before biological break- 
down puts an end to it; no doubt the maximum age in this sense is 
seldom reached. By the age of maturity is meant the age of sexual 
maturity, for an animal should be considered as mature only when it 
is capable of reproducing itself. It varies among species, but most 
common birds reach sexual maturity in their first year of life. There 
appears to be considerable "biological latitude" in age of sexual 



maturity. The Jungle Fowl breeds at 1 year of age, but some Domestic 
Chickens will lay fertile eggs at 6 months and sometimes even at 4 
months of age. The maximum breeding age has little practical signifi- 
cance in nature, for it is evident that few birds arc successful enough 
in coping with the vicissitudes of life in the wild to outlast the capacity 
for reproductive life. 

Life Expectancy. As yet we have few data on the life expectancy 
of wild birds. The mortality of young birds may reach as high as 
75 per cent during the first year and continue henceforward at the 
rate of 45 per cent per year. But it varies among species and perhaps 
geographically as well and may reach 60 per cent in some and be lower 
in others. The life expectancy of the American Robin when fledged 
seems to be about 14 months. The life expectancy of the Herring 
Gull when fledged, on the other hand, seems to be about 30 months. 
The average fledged Grouse or Quail has a life expectancy of about 
1/2 years. A study of the Ovenbird (Ilann, 1937) shows that the 
female may have a life expectancy of about 29 months and the male 
of about 34 months. (Yet there are indications that the female among 
birds tends to outlive the male, just as in man.) 

Longevity. The term longevity has several shades of meaning for 
various writers, but basically it means the duration of natural life. 
A popular rule of thumb gives the age an animal should attain as six 
times the number of years needed to reach maturity. We do not 
know how true any rule of this sort is for mammals, and we know 
less of its truth in birds. The Raven, which matures at the age of 1 
year, for example, may live a decade or more. 

Known advanced ages among birds in the wild are few. The 
Ovenbird has been reported to live 7 years; 7 years has been reported 
for the Yellow Warbler, Myrtle Warbler, Pine Warbler, and 
Yellow-throat. It may well be, therefore, that 7 years is the "thrce- 
scorc-and-ten" for Warblers and other small birds. 

Banded Cardinals have reached 14, 16, and 21 years, respectively, 
and banded Mallards have reached the advanced age of 15 years in 
the wild. Reports of age in wild birds are subject to error unless the 
birds have been marked for complete identification, but recognizable 
individual Herring Gulls have lived 24 years, the Black-backed 
Gull 63. 

Records of age among birds in captivity suggest the potential age 
of birds. A bird that successfully adapts itself to life in captivity con- 
ceivably could live out the potential life of the body mechanism. The 
uncertainties and dangers of life in the wild exceed those of birds in 
the zoo; yet we must not forget that captive birds live artificial lives. 


As we are interested at the moment not in natural life of the caged 
bird but in the lasting possibilities of the avian body mechanism, we 
can overlook the artificiality of captivity. Tame Parrots have been 
reported to attain great age. Definite evidence indicates that an Eagle 
Owl has lived to 68 years in captivity (Griswold, 1953), which is the 
all-time bird record. Other reported ages are: Condor, 52 years; 
Pelican, 40 years; Passenger Pigeon, 29 years; Rose-breasted Gros- 
beak, 18 years; and the Cardinal, 21 years. 

The actual age reached by birds in the wild varies from these 
examples of long life. The expected longevity for several species has 
been calculated from returns of banded birds and mortality rates. 
Table 8*1 gives some examples of mean natural longevity of Pas- 
serine birds. 

Table 8-1 
Mean Natural Longevity of Some Passerine Species 

. Longevity Longevity How 

b P ecies (Dated from) (Years) Obtained* 

American Robin First Jan. 1 1.3 B 

American Robin First Jan. 1 1.4 C 

European Blackbird First Jan. 1 1.9 B 

Song Thrush First Jan. 1 1.6 B 

British Robin First Aug. 1 1.1 C 

European Redstart Breeding Season 1.1 C 

Song Sparrow April 2.0 C 

Song Sparrow April 1.9 B 

Starling (England) First Jan. 1 1.6 B 

Starling (Netherlands) First Jan. 1 1.5 C 

Starling (Switzerland) First Jan. 1 1.1 B 

Ovenbird Breeding Season 1.7 C 

Great Tit First Nov. J 1.4 C 

Great Tit First Nov. 1 1.1 C 

Blue Tit First Nov. 1 1.4 C 

Marsh Tit First Nov. 1 1.6 C 

Rook Nest departure 1.4 B 

* B banded young recovered at death; C calculated from mortality rate. 

Source: Donald S. Farner, "Age Groups and Longevity in the American Robin: 
Comments, Further Discussion, and Certain Revisions," Wilson Bulletin, 61(1949): 


Composition of a Population. Any population of birds at any 
one time consists of individuals of all ages and of both sexes. The age 
composition varies with species, but consists of three classes: young 
birds, mature individuals, and a few that may be classed as aged. 
Common dooryard birds reach maturity in a year, and often birds of 
the year become indistinguishable from adults by fall. Earlier in the 


season, two general classes, adults and immatures, can be identified; 
still earlier there may have been adults, immatures, juveniles, and 
nestlings. Birds that take several years to mature may be distinguish- 
able by year classes, though this seldom happens. 

Some idea of age distribution appears possible from banding re- 
turns. For Bob-white with a life expectancy of about 18 months, 
the composition of the breeding population has been variously indi- 
cated as, for example, 75 per cent second-year birds, 18 per cent from 
the second preceding year, and 7 per cent from earlier years. Al- 
though losses presumably are heaviest among the weaker birds the 
first-year birds and those past their prime the general proportion of 
first-year to older birds seems to be about 50 per cent, but it may 
reach 60 per cent of the total population. No doubt the differences 
in published reports reflect paucity of study, variations from year to 
year, variations from region to region, and variations with habitat. 
The study of age classes in the various seasons is indeed a most fruitful 
theme of investigation, but one where data are gathered slowly and 

In trying to pin down concepts of the number of birds of each age 
in the population of birds, one must of necessity select some specific 
date as a base of operations. As life goes on all the time, age data are 
continuously changing. Various dates have been chosen by various 
people, some rather abstract like "fall" or "winter," some by calendar 
time. Data for the English Robin, based on the birds estimated to be 
alive on August 1 indicate that two-thirds of the birds arc young 
of the year and about a fifth are from the year before; the rest would 
be of earlier years (Lack, 1943). Using January for the American 
Robin, the birds of the year form about half and those of the year 
before about a quarter of the whole population (Farmer, 1949). About 

Table 8 -2 
Age-Class Percentages of Some Species 


Year Class 






6 7 

Q 9 and 
* Above 

Ring-necked Pheasant . . 
Valley Quail 






Per Cent 
2 1 
8 6 
4 2 
10 8 
6 2 
3 1 
3 1 

3 2 
8 2 
(?) - 
Trace - 

1 2 
I 2 

Herring Gull 

Mourning Dove 


American Robin 

Knglish Robin . . 

Song Sparrow 


two-thirds of the Mourning Doves are birds of the year (Quay, 
1951) but only a third of the Mockingbirds (Michener, 1951). The 
very great difficulties in gathering data make for scarcity of such 
records and for impossibility of forming many generalizations. Table 
8*2 gives data as variously reported for several species. What they 
indicate other than that young birds outnumber older birds, and the 
older the birds, the fewer there are, is difficult to say. Some day more 
can be said. Persistence of Mockingbirds into the 7-, 8-, and 9-year 
age classes is rather surprising as compared to other small land bfrds. 
Because the Herring Gull takes several years to become fully adult, 
it would be expected that there would be more birds in the older age 
classes than for a bird maturing in one year. 

Seasonal Changes in Age Classes. In addition to the age classes 
with respect to the population as a whole, a marked seasonal pattern 
of age classes occurs. The proportion of young in the general popu- 
lation rises markedly during the breeding season when it reaches its 
peak for the year. It declines progressively, though not necessarily 
at a constant rate, to about the starting number by the next breeding 
season. Some variation can occur from year to year; probably it does 
rather often. A bad nesting season for a species might locally result 
in almost no production of young birds. Accordingly, the adult- 
young ratio would be high for the adults and low for the young. Yet 
in the long run, the over-all ratio should remain relatively fixed for a 
species (perhaps also for a region). 

Counts of the number of young and adults are rather more easily 
obtained than those measuring the actual age of the birds. The several 
techniques for identifying young birds (that is, birds of the year) 

Table 8 -3 
Adult- Young Ratios Reported from the Field 


Number of Young . 
per 100 Adults Uatc 

English Robin 



er 1 

Herring Gull 


Song Thrush . . ... 




European Blackbird 




Mourning Dove 


Mourning Dove 


Herring Gull ... 


Valley Quail 




American Robin 


American Robin 




have resulted in a number of records of adult-young ratios. That 
they may differ among themselves or with data such as those given 
in Table 8 2 is only to be expected. Differences in reports may arise 
from differences among species, local differences in bird life, differ- 
ences in breeding seasons, differences in sampling methods, differences 
in time of counts, and sometimes plain mistakes. Table 8-3 gives 
examples of ratios of adults and young as reported for various dates. 
The seasonal decline in number will be elaborated further in the 
discussion of life equations. 

~IOO +100 +200 +300 +400 +500 


Fig. 8*1. Survival in the Herring dull and Song Thrush. The data for 
Gulls represent survivors per thousand hatched, for the Thrush, survivors 
per thousand adults. (After Edward S. Deevey, "Life Tables for Natural 
Populations of Animals" Quarterly Review of Biology, 22(1947)383- 

The death rate of adult birds appears to be fairly constant (Fig. 
8-1). For the American Robin, it is said to be about 52 per cent a 
year. For the Mourning Dove, it is estimated to be 80 per cent the 
first year and 55 per cent a year for the next 10 years. But on the 
average, the death rate may be about 45 per cent for each age class. 

Life tables have been used commonly in the study of human mor- 
tality rates, but only a few have been attempted for birds. Various 
starting dates have been used, but the January 1 following hatching 
is, suggested as best. Life tables have been given by Deevey (1947). 
Table 8-4 shows a representative life table for the Herring Gull. The 
life expectancy in years after August 1 has been reported for several 
species: Lapwing, 2.36; Herring Gull, 2.44; English Blackbird, 1.58; 
Song Thrush, 1.44; English Robin, 1.01; Starling, 1.49. 

Age Indicators. Birds have a determinate type of growth which 
renders difficult the telling of age, but some useful indicators are 
available for bird study. The skull in young Passerine birds, for ex- 
ample, requires about 6 months for complete ossification, so that the 







^ "D 


CO T> 














Mortality Rate 
per Thousand 
Alive at Start 
of Age Interval 

to*o fa 



o bo P-> 

i i 










skulls of young birds in summer or early fall have a clearer and softer 
appearance than those of adults. Later this gives way to the granu- 
lated and hard appearance of the adult. A Passerine skull with in- 
complete ossification indicates a bird of the year. 

The bursa of Vabricius can be used until about midwinter (some- 
times longer) for separating old and young gallinaceous birds, Water- 
fowl, and probably some others. The bursa usually has disappeared 
or become closed in adults, while that of the young remains open and 
measurably deep (Fig. 8-2). The bursa may close in late summer 



Fig. 8 "2. The bursa of Fabricius disappears m adults and for many 
birds can be used to indicate age. (By permission frow The Ducks, Geese, 
and Swans of North America, by Francis 'H. Kortright. Copyright, 1942, 
American Wildltfe Institute, Washington, D. C.) 

among Passerine birds. The spur of birds like the Pheasant requires 
time to develop and is smaller in the younger birds. The bill of the 
young Woodpecker feels noticeably sharper against the hand than 
that of an adult which has been dulled by chisel-work. 

Young birds have plumages somewhat fluffier than adults, but it 
may take considerable experience to recognize the difference. Some 
birds have immature plumages which last beyond the first year. The 
Bald Eagle, for example, takes at least 3 years and perhaps 4 or 5 
years to obtain its white head and white tail. Obviously, therefore, a 
Bald Eagle without the white head or tail would be less than 3 or 4 
years of age and one with it would be more. In the same way, the 
plumage of the Herring Gull requires 3 to 4 years for complete 
development. The plumage of each year's age class differs somewhat 
from that of others, so that by careful study a Herring Gull popu- 



lation could be divided into one, two, and three, or even older age 
groups with considerable accuracy. We readily recognize the plum- 
age of first-year birds like the American Robin, American Egret, and 
Red-winged Blackbird. Sometimes remnants of immature plumage 
persist 2 years or more, as in the Rose-breasted Grosbeak, Crossbill, 
Purple Finch, and sometimes in the Redstart. Young birds may retain 
a few juvenile feathers, e.g., the vestiges of crossbars or edgings in 
primary coverts of the Mourning Dove, Bob-white, and Hungarian 
Partridge (Fig. 8-3). 

Fig. 8*3. The edgings on the wing coverts of this Bob-white indicates 
a bird of the year. 

Fig. 8 4. The outer two juvenile primaries of gallinaceous birds are 
retained until the first post breeding molt, which indicates a young bird 

Primaries usually molt from the inside outward and secondaries 
from the outside inward (Chapter 7). Since molt is generally progres- 
sive, tables and charts can be constructed to indicate age during the 
first summer by progress of wing molt. The ninth and tenth primaries 
(outer) of young gallinaceous birds are carried through to the fol- 
lowing fall, while all those of adults are replaced in the same molt, so 



that young birds in the fall and spring have wing feathers of two 
different plumages. The ninth and tenth primaries represent the 
juvenile plumage and the rest represent the first-winter plumage. 
The two outer primaries of the young look more pointed and worn 
than the primaries of adults, which character is useful as an indicator 
of age until the fall of the year following hatching (Fig. 8 '4). 

The flight and tail feathers of young Ravens and Crows and per- 
haps all members of the Corvidae do not molt the first fall. The tail 
and wing feathers thus will appear more worn in first-year birds and 
will have a brownish cast rather than the glossy black of the adult. 

(a) (6) (c) 

Fig. 8*5. Left. The down of some Waterfowl (a) breaks off to leave 
a notch (b) that indicates a young bird in contrast to the adult tail feather 
(c). (By permission ]rom Ducks, Geese, and Swans of North America, 
by Francis H. Kortright, p. 26. Copyright, 1942, Wildlife Manage- 
ment Institute, Washington, D. C.) Right. The irregular wear and shape 
of the outer tail feather of the young Crow (b) contrasts strongly with 
that of the adult (a). (After J. T. Emlen, Jr., "Age Determination in the 
American Crow," Condor, 38(1936):101.) 

The wing length of adults is also greater than that of the young. The 
ventral ridge of the barbs lacks pigmentation in the young as com- 
pared with those of the adult. The ventral part of the rachis of the 
wing feathers is whitish nearly to the tip in first-year birds. The tail 
itself appears rounded in first-year birds but the tail of second-year 
birds has a squarish appearance (Fig. 8-5.). 

The down feather in Waterfowl, as in other birds, grows from the 
same feather follicle as the succeeding feather, which carries the down 
feather on its tip as it pushes out. In the tail of Waterfowl, the down 
breaks off and leaves a notch, often easily recognized, which denotes 
a bird of the year (Fig. 8-5). The tail and wing feathers of the 
young have a dull appearance also. 



The feet and legs of adult Pigeons and Doves bear heavier scales 
than those of young birds; the feet of the young also have a pinkish 
appearance. The feet and legs of older Mallards, Shovelers, Black 
Ducks, and Mergansers are brighter red than those of the young. The 
undersurface of the claws in very young Crows has a horn color. 

The color of the iris may at times be usable for distinguishing 
age, although we know comparatively little about the color of the 
iris, just as we know little about many other color characters in birds. 
The iris in very young Crows is blue; it is yellow in the adult Eagle 
but brown in the young. 

Poultry handlers have developed a rapid technique for distinguish- 
ing the sex of day-old chicks by inverting the cloaca to show the 
genital eminence in the male chick or its absence in the female. Skilled 
operators can sex upward of a thousand an hour with better than 
95 per cent accuracy. With practice, the technique may be applicable 
to nestling birds. 

Dorsal Side 
Bursa ^ 




Adult male 

Adult female 

Ventral Side 

Fig. 8 6. The presence of a sheathed penis indicates an adult wale in 
the Waterfowl. The visible opening to the oviduct Indicates an adult 
female. The burs a of Fabricius indicates a young bird. (By permission 
from Ducks, Geese, and Swans of North America, by Francis H. Kort- 
right, p. 3f. Copyright, 1942, Wildlife Management Institute, Wash- 
ington, D. C.) 



Adult Waterfowl can be distinguished from the young by the 
sheathed penis of the adult male and prominent oviducal opening 
of the adult female. This will indicate sex at the same time (Fig. 8-6). 

The bill in male Red-winged Blackbirds may be used also as an 
indicator of age. The young have a short and stubby bill which 
grows longer with age (Fig. 8-7). Because the bill of these birds has 
been used also as a taxonomic character, only birds of the same sub- 
species can be used comparatively for age determination. 

Fig. 8 % 7. Age variation In ihe Nevada Red-winged Blackbird, (a) 
Adi/Its, (b) one-year old, (c) juvenile. (After A. J. Van Rossew, "The 
California Forms of Agelaius phoeniccous (Linnaeus)" Condor, 28(1926): 

Young of the California Gull have a black bill that becomes pro- 
gressively yellower as the bird matures. It takes about four years to 
reach the adult stage of a yellow bill with a small red spot on the upper 
and lower mandibles (Fig. 8-8). Changes in the Herring Gull are 

pinkish white 


bluish white 

pale yellow 

orange red 



orange red 


bright orange red 

or vermilhon 

Fig. 8*8. The bill of many Gulls varies 'with age, as shoivn in these 
diagrams of the bill of the California Gull. (After David W. Johnson, 
"The Annual Reproductive Cycle of the California Gull, Condor, 58 



A large assemblage of information on sex ratios of game birds has 
already been accumulated; these data suggest widely different varia- 
tions in time, place, and species (Mayr, 1939). In theory, at least, 
the primary sex ratio (at fertilization) is equal. Should there be no 
differential mortality in the oviduct or during incubation, the seq- 
ondary sex ratio (at hatching) should also be equal. Were there no 
differential mortality in life, the tertiary sex ratio (postnatal) would 
also be equal. The secondary sex ratio in most birds may show a 
slight excess of males, a condition paralleling that for mammals. The 
ratio for the Domestic Fowl is given as 97: 100.* We must consider 
the whole matter of bird sex ratios as subject to very great error. 
Some indication of the ratio of males to each one hundred females 
appears in Table 8-5. 

Table 8-5 
Reported Ratios of Males to One Hundred Females 

Species Ratio Species Ratio 

Boat-tailed Crackle 44 Herring Gull 106 


Boat-tailed Crackle 52 Mallard Ill 

Rock Dove 105 Pintail 134 

Domestic Fowl 97 Canvas-back 103 


Boat-tailed Crackle 53 American Crow 124 

European Blackbird 67 Mallard 109 

Song Sparrow 105 Pintail 55, 288 

Chiff-chaff 177 Baldpate 114 

Starling 212 Canvas-back 85,195 

Bob-white 114 Dabbling Ducks Ill 

American Coshawk 61 Diving Ducks 161 

Eastern Cowbird 248 New World Warblers (average) 119 

Valley Quail 112 Mourning Dove (adult) 128 

Mourning Dove (immature) 113 

Observations indicate some consistent variations, though we can- 
not interpret their entire significance. Studies of Waterfowl indicate 
that males consistently outnumber the females. Much variation among 
species and among various groups of the same species in different areas 

* One method of indicating sex ratio is as a ratio of males to a hundred females, 
which method is used here. 


has been reported. But studies of sex ratios in broods seem to indicate 
ratios more nearly equal than in adults. They suggest either a rather 
higher mortality ratio among females or important geographic segre- 
gation. The hatching ratio of the Canvas-back in Manitoba, for ex- 
ample, has been reported as 103 males to each 100 females. But male 
ratios of 195 (spring, Manitoba), 85 (spring, Manitoba), 85 (fall, 
Manitoba), 210 (Washington), 450 (Louisiana), and 205 (Louisiana) 
indicate the very real difficulty of determining the true sex ratio of 
so wide-ranging a bird (Yocom, 1951). 

The Honey-eater family (Atelipbagidae) has a genus (Myzomela) 
of the Australian region in which the males outnumber the females 
by reported numbers varying from about 105 males to each 100 fe- 
males to more than 1,000: 100. A related genus (Lichmera) is reported 
to have males outnumbering the females 8 or 9 to 1 (Mayr, 

Variations in sex ratio may have some correlations with the life 
history of the species, but evidence of such relationship has not been 
developed satisfactorily. Concerted effort over the whole range of a 
species whose sex may be readily recognized in the field, coupled with 
consistent local counts of adults and young, seems essential for pic- 
turing sex ratios adequately. 


*ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 

* ALLEN, GLOVER H., Birds and Their Attributes. Boston: Marshall Jones Co., 1925. 

BROWN, C. EMERSON, "Longevity of Birds in Captivity," Auk, 45 (1928): 345-348. 

FLOWER, STANLEY SMITH, "Further Notes on the Duration of Life in Animals IV 
Birds," Proceedings of the Zoological Society of London, Scries A (1938): 195-235. 

MAYR, ERNST, "The Sex Ratio in Wild Birds," American Naturalist, 73 (1939): 156-179. 

MITCHELL, P. CHALMERS, "On the Longevity and Relative Viability in Mammals and 
Birds, with a Note on the Theory of Longevity," Proceedings of the Zoologi- 
cal Society of London (1910:425-458. 

RICHDALE, L. E., Sexual Life of the Penguins. Lawrence, Kan.: University of Kansas 
Press, 1951. 

RICHDALE, L. E., Study of a Group of Penguins of Known Age. Dunedin, New 
Zealand: Privately published, 1939. 


Evolution in Bird Life 

We can hardly discuss the relationships of any species or its envi- 
ronmental adaptations and distribution without frequent reference 
to evolution. Indeed, even interpretation of behavior in the individual 
often requires reference to the workings of evolution. The scarcity 
of birds in the fossil record precludes demonstrations of relationships, 
changes, and trends such as have been made with some mammals and 
reptiles. But the large fund of knowledge of living birds (larger by 
far than the corresponding knowledge of other groups) lays before 
the student a great array of evolutionary results. These illustrate well 
many of our concepts of evolution and evolutionary processes as they 
operate in the living animal. Especially is this true of nonstructural 
characters, which have attracted the attention of field students. 

Evolution has been defined as slow, inheritable changes through suc- 
cessive generations. The fewer the number of generations between 
animals and their common ancestor (or perhaps more correctly their 
common ancestral population), the closer do we consider the relation- 
ship. But because this relationship cannot be established by counting 
generations, as in the family relationships of man, naturalists customar- 
ily use degrees of difference and similarity as indicators of relationship 
(Chapter 2). 


Characters Undergoing Evolution. Although all inheritable char- 
acteristics of a bird are subject to modification, evolution apparently 
has not followed all the avenues theoretically open. Some characters 
have received considerably more evolutionary attention than others. 
The trend in the Galliformes, for example, seems to have been for 
greater selection of terrestrial characters and that of the Anseriformes 



for improved aquatic adaptation. It would be theoretically possible 
for some gallinaceous ImeTto "have evolved in an aquatic direction and 
for some Anserines to have taken on gallinaceous habits. (The ter- 
restrial habits of some, such as the Nene Goose (Hawaii) or the 
Falkland Goose, can hardly be called true gallinaceous traits.) The 
restriction of the evolutionary range to certain lines in various species 
or groups has been explained by mutations in genes, the genes perhaps 
being chemical substances limited in their reactions by molecular 
structure. Such chemical limitations would thus determine the subse- 
quent range of variation expressed by the characters under generic 
control. But a fundamental weakness (and to some nongeneticists an 
insurmountable flaw) in the evolution-by-mutation concept is the 
field and laboratory evidence that mutations are well-nigh universally 
of a weakening nature adversely affecting vigor (page 395). 

However evolution may be controlled or induced, any character 
influenced by one or more genes could conceivably evolve into some- 
thing else. Evolution may involve structure, physiology, behavior, or 
any other inherited trait. Evolved characters may not now appear of 
an adaptive nature, though characters we recognize as being of "sur- 
vival value" do. (Subjectiveness is involved, however, in our recog- 
nition of adaptiveness and survival value.) In a sense, various species 
of birds are not necessarily so well adapted to their respective positions 
in nature as might be the case (Chapter 3). The assumption that 
characters evolve for greater "biological efficiency" may not be true 
always. Anyone who has watched a male Scissor-tailed Flycatcher 
flying in a wind alongside a female can readily see the greater handi- 
cap provided by his tail (Fig. 3*7). In the same way, a male Boat- 
tailed Grackle is no aerial match for the female or even for other 
Crackles. Such characters may have evolved for purposes other than 
everyday living. Adaptations may be for different aspects of life: 
what may be an advantage in one aspect may be a minor detriment in 

But many birds appear to get along well even though not perfectly 
adapted; yet lack of critical adjustment seems to spell extinction. In 
a sense, we measure successful adaptation to environment by survival 
and by the number of descendants produced, in the latter cases prefer- 
ably perhaps on a biomass basis. But this may not be altogether sound; 
were we able ifflTieiKJllfe 1 "Bit 'biomass (page 256) on the basis of 
ecological occupation, we would probably be on sounder grounds. A 
grassland bird found to occupy 90 per cent of its available living space 
might be considered proportionately more successful than a more 
numerous brushland bird occupying but 70 per cent of its living space. 

The great bill of Toucans seems to have no "adaptive explanation," 


and it may doubtfully suggest one of the lines of evolutionary effort 
open in the Toucan inheritable make-up that has been persistently fol- 
lowed (Fig. 9- 1 ), perhaps as an avian example of orthogenesis. While 
it seems to be a rule that characters not of survival value tend to drop 
out, it may be that "neutral" characters like the unusual Toucan bill 
do not follow such a rule. We must recognize, however, our incom- 
plete knowledge of the relationship of the Toucan bill to the bird's 
total behavior pattern. Like the tail of the male Scissor-tailed Fly- 
catcher, it may serve as a social releaser. 

Fig. 9 I . The great bills of the various Toucans have no recognized 
adaptive character for everyday living. Their excess development 
(hypertely) may be related, however , to some such function as social 

Adaptations and Evolution. The adaptive nature of bird life 
forms one of the marvels of natural history, and the preservation of 
common traits in related birds, which they no doubt have inherited 
from common ancestors, forms still another marvel. Volumes have 
been written on these two subjects, but many are the pitfalls besetting 
our thinking on the subject of survival and adaptation. The greatest 
loss of the bird population is in the young, and characters deemed 
"adaptive" by biologists seldom are those of the young. Yet to say that 
a character has a survival value need not necessarily imply natural 
selection at work (Chapter 3). Additionally, there may be no such 
thing as "nonadaptive variation"; our lack of knowledge may be re- 
sponsible for the idea. 

The selective action of the environment determines which in- 
heritable variations survive. Two trends may be recognized in this 
selective process: change of the species with time, and increase in the 
number of species in space. In a sense, bird evolution is a process 
whereby those most efficiently adapted to use the biological energy in 
the environment prosper at the expense of less efficient ones (page 
199). It must be recognized, however, that changes in the environ- 
ment may result in additional and other evolutionary readjustment in 


Birds having genetic traits differing from others within a species 
may evolve along separate lines through the workings of isolation. 
The large number of island forms testifies to the effectiveness of geo- 
graphic isolation in concentrating variations and bringing about recog- 
nizable taxonomic differences. Isolation may take forms other than 
geographic separation; ecological isolation seems to be rather common 
and probably a factor in evolutionary changes. The greater variation 
toward the periphery of a species range probably reflects both geo- 
graphic and ecological influences. Reproductive isolation, however, 
by whatever means accomplished, is the ultimate isolation. 

Evolution in the More Recent Past. The similarities of skeletal 
structure between Miocene and Recent birds are remarkable (How- 
ard, 1947). There seem to have been no great changes in bird life, 
except extinction of species, for a long time past. The evolutionary 
trend since the Pleistocene shows consistent characteristics in some 
groups. In the Pleistocene deposits of the famed Rancho La Brea 
(which deposits may have had selective action), only about 10 per 
cent of the birds represented are reported to be of extinct species, in 
contrast to more than 40 per cent of the mammals. The ratio of the 
extinct to the living in the predatory birds, on the basis of numbers 
of individuals, is 23.8 per cent; in mammals the ratio is 95.1 per cent 
(Howard, 1930). More than half the fossils belong to families of the 
Falconiformes (Fig. 9-2). 

The fossil record indicates that the time since the Pleistocene (and 
probably Pleistocene also) has been a period of little evolution of 
species. But in passing such judgment upon recent evolution, one 
should not overlook the short span of geologic time involved. The 
Pleistocene and Post-Pleistocene times, however, seem to have been 
periods of considerable extermination of bird life. About 1 5 per cent 
of modern North American species are already known to have lived 
in the Pleistocene. It seems likely that all species of today were here 
then, along with many no longer present. It is suggested that all in 
all, a much richer bird fauna may have existed in those earlier days 
(Wetmore, 1933). Yet bird distribution has clearly changed; only in 
8,000 of the past 80,000 years have there been any arboreal birds north 
of Germany (Moreau, 1954). 

Among the birds becoming extinct in the Pleistocene or since are 
some of more than usual interest. The giant condor-like Teratornis 
menmim is known from the California and Florida deposits. At 
least one extinct Turkey (Meleagris trident) lived along with the 
modern Turkey, but we do not know what constituted their respec- 
tive habitat requirements. We should, of course, expect northern 
birds to move equatorward with the advance of the continental ice 




Fig. 92. 77^e relative alnmdcmce of individuals m each family of birds 
recorded fro?!? the Rcmcho La Brea deposits of the Pleistocene. (After 
Hilde garde Howard, U A Census of the Pleistocene Birds of Rancho La 
Brea front Collections in the Los Angeles Museum" Condor, 32(1930): 

cap. Yet many birds now found farther south lived in the southern 
states. In the Florida Pleistocene deposits, for example, are found 
bones of the Jabiru, Mexican Turkey, and Wood-rail, none of which 
now ranges that far north. The California Condor also reached 
Florida and probably had a continuous range between Florida and 
the West Coast. 

Behavior Evolution. There can be no doubt that behavior charac- 
teristics have evolved just as have structural ones. The fact that 
members of a group, such as genus, family, and even order, have char- 
acteristic behavior patterns shows the influence of evolution (see 


Chapter 10). Many of these are adaptive in nature, but others do not 
appear so on the basis of our present knowledge, except perhaps in a 
general way. 

Members of the Holarctic Paridae use variations of a call recognized 
in its best-known rendition as chick-a-dee. Yet the Paridae, from the 
survival standpoint, seem no better off with this type of note than 
the Fringillidae with their chip] or the soft Parulidae tsip. The need 
for a distinctive species call-note is self-evident, but it could take many 
forms, and the similarity within the family can be interpreted reason- 
ably as evolutionary modifications of an ancestral call. In the same 
way, the Paridae live in the woods and only a few have radiated into 
other habitats; the family association with forest is quite likely an 
ancestral one. 

The rather more plastic Fringillidae (sec Table 9 1 ) show marked 
preference for brushy areas or ecological substitutes. But within the 
family, adaptive radiation has superimposed upon the presumable 
ancestral trait modifications that mask the original. Evening Gros- 
beaks prefer large timber and large timber tracts, as do also the 
Crossbills. Most members of the subfamily Carduelinae, to which 
these belong, show the same trait, and from this we can postulate that 
a common ancestor (or ancestral group) evolved the habit and passed 
it on down through its descendant species. In like manner, the Rich- 
mondeninae subfamily has its characteristic habitat selection, largely 
tall brush. The third subfamily, Emberizinae, largely stays with the 
brush or its ecological equivalent, heavy herbs; some have come to 
prefer short trees and some rather open grass areas. The trait has 
become almost lost in some like the Vesper Sparrow 1 , which, even 
though it nests on the ground, prefers hayfields, cropland, and fence 
rows but eschews large fallow fields. 

k% ^ Prey Selection. The selection of prey by avian predators illus- 
trates the -workings of adaptive survival and other processes. It has 
been shown that, under experimental conditions (page 78), Owls 
catch mice concealingly colored significantly less often than those not 
so well matched to the background (Dice, 1945b, 1947). The Owls 
most capable of catching concealingly colored mice would appear 
to have the best survival chances, just as the least conspicuous mice 
would also. We can logically expect that concealing coloration 
operates in the case of small bird prey as it does for mice and in the 
case of other predators as it does with Owls. 

Cploration of Young and Nest, An ingenious ornithologist study- 
ing irircb-ia--the^dry- -Great-Basin noted the tendency of the nestling 
down to be light or dark according to nest site and for the nest lining 



to vary somewhat the same way, though the latter was not so clearly 
evident (Fig. 9-3). This relationship is explained as a device combin- 
ing habit and structure to protect the young from sunlight (though 
it could be explained also as protective coloration). The lighter down 
and lighter nest lining of the species having exposed nests reflect the 
solar rays and thereby aid heat toleration. Species that live under 
darker or less exposed conditions have dark down and dark lining, 
which enable them to absorb and take advantage of the sun's warm- 
ing rays. 






Nest reading vsdown color 

Nest reading 
vs site 


Fig. 9*3. The nest lining of exposed ??ests tends to be lighter than that 
of covered ones. The data are from fifteen different species of birds. 
(Adapted from data by ]ean A. Linsdale, "Coloration of Doivny Young 
and of Nest Linings," Condor, 38(L936):lll-lll.) 

The two responses indicate habit evolution in the adult, which 
selects the nest site and the nest material, and structural evolution 
in the young, which grows the down. A combination of adult and 
young evolution is involved. In some species other ways of counter- 
acting harmful heat rays have been noted, chief among them shielding 
the young with the body of the parent. In such cases, however, down 
and nesk lining do not necessarily show lightness with increased ex- 


Evolution and Ecological Stress. It seems entirely plausible that 
world periods of great stress, particularly climatic, should be periods 
of great evolutionary activity. The fact that the bird's feather cover- 
ing and constant body temperature make it possible to inhabit cold 
regions without hibernation may have reduced the importance of stress 
periods as compared to other animals. In the same way, the body 
covering may protect also from tropical heat and sun so that 
anomalous though it may be birds have become well adapted to heat 
and cold by the same insulating feature. 


An ecological condition favoring bird evolution is the presence of 
suitable niches into which a bird may move by evolutionary adapta- 
tion. It is suggested that when two species meet in the same area, 
individuals that vary most from the general type have a selective ad- 
vantage, particularly if the difference is in feeding structures or in 
habits. This suggestion has been invoked to explain the evolution of 
island birds after invasion, such as of the Hawaiian Honey-creepers, 
as well as in other types of island bird life (Amadon, 1947). Succes- 
sive invasions of an area by the same species may establish additional 
species if the invasions are far enough apart in time so that descendants 
of previous invasions have evolved into distinct and separate species. 
But if the invasions are too close, the interchange of genetic charac- 
ters, through interbreeding, will prevent evolution into new species. 

Shark Fish Seal Penguin 

Fig. 9*4. Penguins (and Hesperornis) of the bird world show adaptive 
convergence for marine life as in seals and other oceanic vertebrates. 
They have the streamlined bodies and paddle-like Imibs of sivmmiing 

Isolation has had marked influences in the bird world, an example 
being the Penguins (page 37) of the Southern Hemisphere (Fig. 
9*4). But isolation does not always favor the bird. About 97 per 
cent of the forms becoming extinct in the past two centuries lived on 
islands, which have been said to serve as areas of genetic selection and 
as evolutionary traps. Some 18 per cent of the natural avifauna of 
Hawaii has been reported as extinct and about 1 1 per cent of that in 
New Zealand. But the only evidence that these extinct birds were out 
of adjustment with their times came after man's interference. The 
assumptions of ornithologists, therefore, may be open to some question. 

Evolutionary Succession. That birds have risen and fallen to ex- 
tinction in the past seems axiomatic. Excellent studies of the Rancho 
La Brca remains show the succession in the Cathartine dynasty (Fig. 
9 '5). Vultures live today, three species in the United States and 
Canada and three in Central and South America. The fossil record 
testifies to the principle of racial senescence. In the Pleistocene, the 
modern Black and Turkey Vultures were ascending in abundance and 
distribution (which is still going on) and the Condor already declin- 
ing. Several other species became extinct before the Pleistocene and 




Fig. 9 5. Succession in the Cathartine dynasty showing abundance and 
geologic history of the Cathartine Vultures. (After Loye Miller, ^Succes- 
sion in the Cathartine Dynasty;' Condor, 44(J942):212-213.) 

a few persisted almost to historical times. A phylogenetic tree show- 
ing the probable relationship among the Vultures in another manner 
is given in Fig. 9-6. 

Ecological Habits and Species Formation. The Blue Grouse of 
western North America has developed into two groups of subspecies, 
a coastal (fuliginostts, sierrae, and hoivardi) and an interior one 
(obscurus, richardsoni, fle???wgi, and pallidus). The birds winter in 
the high country and descend to lower levels for breeding; only along 
summits as in the Cascade Mountains do the two groups come into 
contact. With the coming of spring, each descends its own slope 
for breeding (page 297), so that each is isolated from the other. Were 
it their habit to ascend in the summer and descend in winter as cus- 
tomary for other animals, such isolation would not occur. Hence, 
two different species may be in the making, just as others have devel- 
oped in the past (Fig. 9-6). 

An overcrowded population suffers from intraspecies competition 
ahtrife, t which does not appear necessarily to be in the best interests 
of the species, even though it does influence the survival rate and in 
that way perhaps contributes to evolutionary effort. "Underpopula- 
tion," on the other hand, has unfavorable social influences, and these 
also may bear upon the survival rate. It seems possible that an under- 
crowded organism (if such ever occurs except temporarily) or one in 
small and scattered numbers must face a rather vigorous environ- 


Cathartes Coragyps Sarcoramphus Gymnogyps Vultur 

aura atratus papa californianus gryphus 






Gymnogyps Breagyps 
amplus / clarki 

kernensis i 

\ ^Phosmogyps Palaeogyps 
\ patritus prodromus 

\ i 

\ i 

N Pleisiocathartes 

i Teratornis mernami 
i Cathartornis graci/is 
i i 



Bonasa Canachites Dendragapus Lagopus Pedioecetes Tympanuchus Centrocercus 


Fig. 9-6. (a) Phylogenetic diagram showing possible relationships of 
Cathartid genera. (After Harvey I. Fisher, "The Skulls of Cathartid 
Vultures" Condor, 46(1944):294.) (b)Relationships among the Ameri- 
can Tetraonidae, based upon courtship characters. (After Leonard W. 
Wing, "Drumming Flight in the Blue Grouse and Courtship Characters 
of the Tetraonidae," Condor, 48(1946):151.) 

mental life if it is to progress. The rigors of life may act in this case 
as a molding force, just as crowding may among dense populations. 


The newly hatched bird does not resemble its parents except in a 
general way and must go through a developmental period, occasion- 
ally of several years, before it does match its parents. Many of our 


commonest birds, even the male House Sparrow in our streets, take 
on a winter plumage differing from that of the summer. Acquisition 
of a bright breeding plumage is a fairly common practice. The white 
plumage worn by the Ptarmigan in winter differs more from the gray 
or brown of the summer than from parallel plumages of other species 
of Ptarmigan. 

Many of the variations involving the individual will be considered 
in Chapter 20, which treats of heredity in the bird world (see also 
Chapter 10). But whether these inheritable variations will become 
stamped upon the population character is said to be governed in part 
by restrictions on the flow of genes between adjoining bird popula- 
tions. Birds isolated, as on islands, or restricted in their movement, as 
in isolated habitats, or of low mobility may concentrate a genetic 
character differing from their neighbors and so become marked 
groups (though not necessarily of taxonomic standing). 

Some very interesting evolutionary variations have occurred in 
many parts of the bird world. The Ptarmigan of the Scottish moors 
(known as the Red Grouse) no longer changes to a white plumage in 
winter, thougfTte relatives do. In Australia and the neighboring is- 
Ian3s7 variations in the plumages of males and females occur in the 
Flycatcher, fetroica multicolor. In Australia and Tasmania, males 
and females wear different plumages characteristic of the sex; the 
sexes are alike in Samoa but the plumage is typical of the male in the 
other areas; in part of the New Hebrides, however, the plumages are 
still alike but the plumage is one like that of the female elsewhere. 

Some habits may be explained better as traditional rather than 
hereditary. Tradition may be defined as a habit passed along from 
.generation to generation by association. Though migrating south in 
the fall is an inherited trait, we may consider it traditional for the 
Whooping Crane to winter on the Aransas Peninsula in Texas and to 
stop in migration at certain places along the Platte River. Each young 
bird has learned of these two places by accompanying its elders. In 
the same way, Falcon eyries and even Tit nesting holes have been oc- 
cupied by successive pairs of birds year after year for more than a 
century. The later occupants, however, were far removed in time 
from the first ones. It has been reported that Peregrine Falcons still 
nest on ledges from which falconers obtained birds in the Middle Ages. 

Continuous and Discontinuous Variation. Few species live as 
one continuous group; the majority have been shown to break up 
into races and sometimes into species groups. In the same way, there 
are few species without some geographic variations of habit, song, or 
behavior. No doubt many hidden variations exist also. In a sense, a 
species consists of a "collection of populations." 



Variations occur more in discontinuous ranges than in continuous 
ones, but all ranges are in a sense discontinuous, for they are formed 
of occupied habitat scattered through an uninhabited matrix. JThe 
ocean is more uniform than terrestrial habitat, for example, and its 
major groups have fewer species and races. The small populations of 
isolated islands make possible rapid turnover and mixing of the entire 
gene complex and thereby greater uniformity than elsewhere. An 
island population may have started from only a few individuals and 
embrace but a portion of the whole genetic variability of a species. 








Number of 

M=36.8 25 


M=38.7 25 

M*38.9 37 

=39.4 14 

Fig. 9 7. Cline of increasing size from ivest to east in ten populations 
of the Wedge-tailed Shearwater as expressed by frequency distribution 
graphs. Dotted line represents weans of wing, tarsus, and culmen in In- 
dian Ocean populations. There is a gradual increase in average dimensions 
of birds from the Seychelles to the central Pacific. (Adapted from Robert 
Cushman Murphy, The Population of the Wedge-tailed Shearwater, p. 
5. American Museum of Novitiates, No. 1512.) 



Fig. 9*8. The head adornment of the Drongo (Dicrurus paradiseus) 
of southeast Asia shows every variation frow no crests to long crests. 
Note increase \rorn the small Bornean subspecies (/) to the large 
Himalayan one (9). (/) Brachyphorus, (2) microlophus, (3) platurus, 
(4) formosus, (J) hypobellus, (6) paradiseus, (7) rangoonensis, (8) 
grandis, (9) johni, (10) lophorinus. (Frow Ernst Mayr and Charles 
Vaurie y "Evolution in the Family Dicruridae (Birds)" Evolution, 

A continuous variation, such as the south to north increase in body 
size or body proportions, is termed a #j?Pfl$?Wj^^ (Fig- 

9 7 ) . Continuous variations of a quantitative nature most commonly 
are ones such as pigmentation, size, relative proportion of body parts, 
and physiological responses. In the Drongo (Dicmrus paradiseus) 
every stage is found between uncrested subspecies at one end and 
crested forms at the other (Fig. 9-8). 

The tendency of many birds to behave somewhat differently at the 
periphery of the range from those in the heartland probably reflects 
discontinuous variations. Often the same trend occurs in poor habitat 
within the heartland. Thus, well-situated Mockingbirds in the heart- 
land do significantly less mimicking of their fellow birds than ill- 
situated ones or those of the periphery. The Chat of the periphery 


shows markedly more shyness and retiring habits than those of the 
main range (Brooks, 1942). Brewer Sparrows occupy the sagebrush 
in parts of the West, but elsewhere they are birds of the brushy road- 
sides, pastures, and even timberline shrubs. 

The variability of habit along the periphery or edge of the range 
seems to parallel other variations to some extent. Striking subspecies 
occur mostly at the periphery of a species range rather than at the 
center (Mayr and Vaurie, 1948). It is said, however, that in any 
major group of organisms, we should look to the point of origin for 
advanced present-day types rather than for primitive present-day ones 
(Murphy, 1936). t - 

Variability of Domestic Birds. The very great variability of do- 
mestic birds indicates the biological possibilities of selection, artificial 
though it is. A "breed" actually represents a homogeneous gene com- 
plex, and perhaps two hundred breeds of domestic pigeons exist, all 
descended from the Rock Dove (Colwuba Hvia) of southeastern 
Europe and Asia Minor. There are recognisable breeds of Chicken, 
many of bizarre appearance, far different from the ancestral Jungle 
Fowl. One of the most unusual of the breeds is the Silky Fowl, so 
named because its plumage appears soft, as though covered with a 
silklike hair. The barbules of the feathers are not provided with 
barbicels, or they are so much reduced that the feather does not con- 
solidate as in the normal Chicken. All the contour feathers seem to 
be either retained down or contour feathers much changed from the 
normal. But such a character in the wild Jungle Fowl would seem- 
ingly be a most unfavorable one for survival. 

Flightlessness. Although the central evolutionary effort of birds 
has been for flight and great modification of structure, physiology, 
and behavior to go with it, some birds have lost the power of flight. 
We can conceive of the flight evolution of birds as leading into a 
niche occupied by the rather inefficient flying reptiles that had to 
give way before the onslaught of the more efficient birds. Because 
air-borne life takes a high degree of specialization, it is not to be 
wondered that birds still dominate it, whereas the land-bound reptile 
gave way on land before the mammal that could compete better with 
only a slightly improved biological efficiency. But just how the 
initial aerial movement of the bird succeeded in the face of the ptero- 
dactyls that we assume may have dominated the sky is difficult to 
perceive. We may presume that the initial approach to aerial life was 
along lines more efficient (or climatically independent) for the bird 
than formerly as well as more efficient than those of the flying reptile. 
Birds probably preyed upon the smaller dinosaurs from the air, and 



perhaps the young and eggs of larger ones, which would assist mam- 
mals and climate in bringing on reptilian downfall. Yet the flying 
reptiles may not have dominated the aerial zone and the attributes 
that go with it in sufficient degree to close all possible niches. 

The large Diatryma of the Miocene shows that even in the past 
the aerial bird had reinvaded the earth-bound realm otherwise left to 


Fig. 9-9. Several giant flightless land birds have Jived, (a) Moa (re- 
cently extinct), (b) Ostrich (living), (c) Cassowary (living), (d) Dia- 
tryma (Miocene). 

the mammal (Fig. 9-9) just as the flightless Hesperorms invaded the 
marine realm earlier. But flightlessness seems to confer no special 
favors upon the possessors, and flightless birds seem to have traveled 
into an evolutionary box canyon compared to their flying relatives. 
Flightless birds are few and none has widespread distribution today. 
The insular and distributionally remote regions of Australia and 
New Zealand are the homes of the flightless Kiwi, Notornis, Emu, 
and Cassowary (Fig. 9-9). 

Flightlessness may ariss in^almpst any group where the inability 
to fly is nQt a .current disadvantage (Fig. 9-10). The Burrowing 
Parrot (Strigops, or sometimes Stringops) of New Zealand has al- 
most no keel, so long has it lived flightless. In the Alcidae, the Great 
Auk survived before man destroyed it; its range, however, could 
hardly be called remote from land masses. On the Galapagos Islands 


occurs a flightless Cormorant at the northern range limit of the flight- 
less Penguins. A flightless Rail, now extinct, lived in the Laysan 
Islands and a flightless Wren in the West Indies (Baldwin, 1947). 
The flightless Grebe reported from Lake Titicaca in South America is 
assumed to have arrived as a flier at some remote period in time. 

Fig. 9-10. Fligbtlessness way occur in isolated groups or remote places 
'where loss of flight appears to confer no fatal disadvantage for the time 
behtg wider 'natural conditions, (a) Dodo (Cohnnbtforwes), (b) Kiwi 
(Apterygtformes), (c) Great Auk (Charadritforwes). 


Evolution and Adaptive Radiation. Many groups of birds show 
rather unusual capacities for adapting themselves to various ecological 
conditions. In fact, active adaptive radiation seems almost like a 
characteristic of some. Yet some show little tendency for becoming 
adapted to several differing ecological situations (Fig. 9' 11). Several 
members of the same family will often penetrate the same type of 
environment; members of several different families may also penetrate 
the same type (Table 9-1). Thus, the forest may be the home of 
birds that belong to such families as Strigidae, Tetraonidae, and 
Fringillidae. Members of these same families occur also on treeless 
tundra. They have thus made parallel penetrations of these major 
ecological types. Birds are particularly noted for this habit (Fig. 
9-12); other animals may do so but not so conspicuously. Because 
of convergence, the resulting, unrelated birds may on occasion appear 
more alike in looks and actions than other more closely related ones. 
Th&y,,may bocomo^ ecological homolognes (page 35). 

At times, adaptive radiation appears to become a high specialization 
that may indicate racial age (irrespective of the number of calendar 
years involved). Among living birds are some characteristics that 
have sometimes been interpreted as indicating racial old age. The 



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Fig. 9 1 1 . Possible origin of the three major icterid lines from an an- 
cestral C oiv bird: (a) Elackbird-Tronpial (agelaiine), (b) Grackle (qiiis- 
calrne), and (c) Cacique (cassicine) lines. (Prom William }. Beccher, 
"Adaptations for Food Gathering in the American Blackbirds" Auk, 

Kirtland Warbler, for example, has become so particular in its wants 
as to occupy a very restricted habitat in a very restricted range. In 
its special adjustment for feeding extensively upon bark beetles, the 
Ivory-billed Woodpecker may show equal restrictiveness. Though 
we can perhaps think of these species as "old," it does not necessarily 
reflect family "senility." The Wood Warblers (Parulidae) surely are 
a "vigorous" family: the Black and Turkey Vultures are also vigorous, 
though the California Condor is not (Fig. 9-5); most Woodpeckers 
would clearly be considered as vigorous species, even though the 
Ivory-billed may be thought otherwise. 

Evolution and Spread. The relation between evolution and dis- 
tribution reveals many workings of evolutionary processes, as is illus- 
trated by the various Chickadees of western and northern North 
America (Fig. 9-13). The Chestnut-backed Chickadee occupies the 
humid Pacific Coast region from the Alaskan panhandle south to 
Monterey, California, in a narrow strip usually fewer than a hundred 





' ' 








f I 


* Thorn-shrub desert # * Alpine meadow 











Fig. 9 1 2. Adaptive radiation results from the evolutionary invasion of 
habitats as illustrated by competitive radiation of various groups into com- 
mon habitats. The larger groups radiate most widely. Genera seldom 
use more than two or three different general habitats, and species seldom 
use more than one. 

miles wide but extending into northern Idaho and adjacent Montana 
(Grinnell, 1904). To the north, the Hudsonian Chickadee has its 
breeding range across northern North America south to the United 
States border. The Alaskan Chickadee lives in the Alaskan region 
and adjacent Canada. 




Interior of 

Alaska and 


British Columbia ^ District 

B District 

Santa Cruz 
c District 

through geographic and faunal space - 

Fig. 9 - 13. Evolution and spread of the Chickadees of western North 
America. (After Joseph Grinncll, "The Origin and Distribution of the 
Chestnut-backed Chickadee," Auk, 21(1904):364-382.) 

Certain characteristics of these birds are apparent. The young 
Iludsonian resembles the parents, while those of the Chestnut-backed 
resemble the presumed general ancestral type more than today's 
adults. The sides of all are rusty when young, though those of 
Pants rufescens barlow are gray when adult, those of P.r. neglectus 
pale rusty when adult, and those of P.r. rufescem deep brown in 
adulthood. The young of P.r. barlow are paler than those of the 
others. The difference in adults follows the Gloger Rule of paler 
color in drier areas, darker in more humid ones. P.r. rufescens occu- 
pies the humid coast, P.r. neglectus the fairly humid Marin district, 
while P.r. barloivi lives in the drier Santa Cruz country. In general, 
Parus rufescens differs from Parus hudsonicus by intensification of 
browns and decrease of size under conditions of moist climate. The 
Chestnut-backed Chickadees pushing farther down the coast have 
become lighter again in the drier climate (Grinnell, 1904). 

In addition to color differences, the Hudsonian Chickadee has a 
shorter tail, which is in line with the rule of shorter tails for birds of 
the tree tops having more powerful flight than those of the bushes and 
lower tree branches. The Hudsonian occupies the tops of trees and 
lives in taller trees more. (In the same way, Mockingbirds, Bush-tits, 


and Wren-tits that flit from limb to limb or from bush to bush across 
short open spaces have developed relatively long tails.) 

Among naturalists, the term endemic is sometimes used in referring 
to birds now living in the place of evolutionary origin. The differ- 
ence between native and endemic (the latter may have a different use 
in medicine) is that a species is native to a place that it occupies by 
its own efforts as distinguished from occupation resulting from acts of 
man. The bird may have evolved at some distance and have spread at 
some time in the geologic past. Thus, two species may be native to 
the same island, but the ancestors of one may have come from the 
mainland, those of the other became a species (or subspecies) on the 
island. The latter would be called endemic. 


DOBZHANSKY, T. G., Genetics and the Origin of Species. New York: Columbia 
University Press, 1941. 

GRASSE, PIERRE-?., Traitc de Zoohgie: Oiseaitx. Vol. 15. Paris: Masson et Cie, 1950. 

GREGORY, WILLIAM K., Evolution Emerging. New York: The Macmillan Co., 1951. 

HEILMANN, GERHARD, The Origin of Birds. New York: Appleton-Century-Crofts, 
Inc., 1927. 

MATHEW, WILLIAM DILLER, Climate and Evolution. New York Academy of Sciences, 
Special Publications, 1939. 

MAYR, KKNST, Systematic* and the Origin of Species. New York: Columbia University 
Press, 1942. 

MAYR, ERNST, K. G. LINDSEY, and R. L. USINUER, Methods and Principles of Sys- 
tematic Zoology. New York: McGraw-Hill Book Co., Inc., 1953. 

NEWMAN, H. H., The Phylum Chordata. New York: The Macmillan Co., 1939. 
*PYCRAFT, W. P., A History of Birds. London: Methucn & Company, 1910. 
*THOMSON, J. ARTHUR, The Biology of Birds. New York: The Macmillan Co., 1923. 
*SIMPSON, GEORC.E GAYLORD, The Major Features of Evolution. New York: Columbia 
University Press, 1953. 


Bird Distribution 

Like so many other matters of nature, bird distribution is a continu- 
ous process. We can see it today largely as an end product from the 
past, save for what may happen in the short span of an observer's 
lifetime or that of the written word. In addition to the natural distri- 
bution, an acceleration of change has been brought about by the 
impact of man himself upon the earth. To the findings of living dis- 
tribution may be added fragments of fact from findings in the fossil 
and subfossil record. 

For the sake of brevity, the many factors influencing the distribu- 
tion of birds may be listed as ecological and historical ones. The 
ecological factor includes both the suitability of the habitat (obvi- 
ously, it must be suitable else the species will not prosper) and the 
development of the environment that makes it suitable (that is, eco- 
logical evolution). The historical factor includes both the develop- 
ment of the bird that it may become adapted to a habitat (organic 
evolution) and development of the land and its attributes (physical 
evolution). Ecological evolution depends to a high degree upon 
physical evolution (and sometimes vice versa). That they overlap 
or merge is another example of the oft-stated axiom that nature shuns 


Needs of a Species. Biologists conceive of each species as pos- 
sessing combinations of needs peculiar to itself and distinct from those 
of every other species. Except for geographical restrictions, such as 
those of islands, the range of each species of bird differs from that of 
all others. And except for possible obligate parasites, the range for 
each species differs from that of all other animals also. 



Although birds by their warm-blooded nature are able to live 
somewhat independent of temperature, the prime element that still 
controls their distribution is a suitable climate. Other needs center 
around such characteristics as cover and the physical make-up of the 
environment, but no doubt many subtle needs also exist. In a sense, 
cover is partly an expression of climate, and the most important in- 
fluence of climate on birds seems to be this indirect one through cover. 
The physical character of the cover in part reflects the topography. 
All in all, so intertwined are these needs of a species that we cannot ar- 
bitrarily delimit them, but we can recognize their operations by field 

An adjustment in the respective abilities and adaptations of the 
Prairie and Peregrine Falcons illustrates the presence of determining 
factors in the selection and maintenance of their respective ranges in 
western America.* Both species have about the same size wing and 
body, but the Prairie Falcon is lighter in weight than the Peregrine. 
Hence, it has a lower wing-load and consequently greater advantage 
in the thinner air of higher altitudes. The Peregrine is a water bather, 
the Prairie a dust bather, which ties the Peregrine to water and aids 
the Prairie in using dry regions. Both feed upon birds caught in fair 
flight, but the relatively greater power of the Peregrine, because of 
greater muscle-to-wing ratio, makes it superior in lowland bird pur- 
suit. The Prairie, however, feeds upon rodents far more than the 
Peregrine, an adaptive adjustment to the dry regions, the lands 
toljere rodents are most abundant. The accuracy in flight of the 
Prairie Falcon defies description. A ground squirrel peeking out of its 
burrow may have but an inch of head exposed as it surveys the out- 
side, but this is a sufficient target for the full-power stoop of a Prairie 
Falcon. The open talons hit the target, the rear toe being the principal 
weapon with the forward talons coming into play like a pair of tongs. 
They hold and bear off a light rodent, but they tear through a heavy 
one, which often lands mortally wounded away from its burrow, in 
position for a return pickup. This coordination for pin-pointing its 
target at high speed may be equally developed in the Peregrine as 
anyone might agree who has seen one capture a White-throated 
Swift but the Peregrine seldom stoops to rodents or birds on the 
ground. Although a few Peregrines nest above 5,000 feet altitude 
and a Prairie Falcon nest has been reported on an island in the Gulf of 
California, the balance in their respective capacities and needs is such 
that the Peregrine generally occupies the humid regions, shores, and 
lowlands. The Prairie Falcon, on the other hand, takes over the dry 

* Dr. R. M. Bond has kindly supplied much of this information from his unpub- 
lished and matchless field observations of Falcons. 


regions and high altitudes. Slight variations in habits and capacities 
between other closely related species may likewise be found. 

A brief survey of the Cliff Swallow colonies in the Sacramento 
Valley of California showed that sixty-eight colonies containing 
some 8,200 birds occupied about 1,600 square miles, the closest col- 
onies being scarcely a half mile distant (Emlen, 1941). All used man- 
made structures (bridges and buildings) though in nature the species 
uses cliffs, usually along streams but always near to water. The main 
requirements for a nesting site in this case seem to have been a pro- 
tected vertical surface, reasonably open terrain, and a nearby mud 

Zoogeographic Laws. The recognized laws of zoogeographic 
distribution may be stated briefly as follows: All animals are found 
wherever conditions are suitable unless (1) they were unable to 
reach an area, (2) they reached an area but were unable to survive, 
or (3) they reached an area and evolved into another species. These 
three principles of animal distribution show clearly in bird life even 
though birds, because of their flight power, have greater mobility than 
other vertebrates. 

What appears to be a case of inability to reach suitable range oc- 
curred in the distribution of the Mountain Quail in the Pacific North- 
west. The indications are that originally it reached north only as far 
as the Columbia River, and that settlers in the region moved some 
birds to the Washington side about the year 1 860, where they have 
since succeeded well. It would appear that the width of the lower 
Columbia River exceeded the flight limit of the bird. Island birds as 
a rule belong to groups on the nearby mainland. Only the strongest 
flyers generally have the power to reach the more distant islands. 
Flight powers and flight limits, it should be self-evident, form one of 
the limitations to spread. 

The second principle has been shown a number of times, though 
chiefly through the fossil records. Trogons now confined to the 
Old and New World Tropics once ranged as far as France in the Old 
World and no doubt occupied the land between. Fossil Condors in 
Florida and fossil Albatrosses in England show wider former distribu- 
tions for these species than is seen now, and evidently they were 
unable to stay after having once become established. Even the excur- 
sions of the Dickcissel into New England and its subsequent retreat 
show evident inability to survive. Dickcissel remains in an Indian 
Pueblo of the twelfth century (Miller, 1940) suggest that once the 
species may have invaded the Southwest (Fig. 10-1). 

The third rule, that of evolution into another form, appears fre- 
quently. Thus the Black-capped Chickadee is believed to have become 



transformed into the Carolina Chickadee in southeastern North Amer- 
ica, just as the Hudsonian probably gave rise to the Chestnut-backed 
(Fig. 9- 13). 

Members of the Tetraonidae seem able to show all three rules in 
operation, as do members of most other families. The members of the 
Brush Grouse group (Bowasa) have not been able to spread into the 
Old World because no suitable brush connection seems to have ex- 
tended across the Alaska-Siberia land bridge.* But Tundra Groyse 

20th century 

19th century 

range extension 

X 12th century 


Fig. 10- 1. The range of the Dickcissel in the twentieth century in- 
cludes chiefly the interior prairie country. In the nineteenth century, it 
reached Neiv England. It reached Arizona (indicated by an X) in the 
twelfth century. 

(Ptarmigan) have crossed over the tundra connection. Members of 
the Tree Grouse group have made the crossing, clearly when tim- 
bered conditions were more favorable than at present. The Spruce 
Grouse crossed at some time in the recent past and became Canachites 
(Falcipenms) falcipewnis. But the Prairie Grouse have not crossed; 
they are birds chiefly of dry interior grasslands, and any coastal grass- 
land in Alaska would probably have been a marine type, humid and 
unsuitable. The Ptarmigan once covered many mountain tops of 

* The Hazel Grouse (Tetrastes) has been said to be related to the Ruffed Grouse 
(Bonasa), but parallel development seems a better explanation. 



western North America, especially toward the more southern parts of 
the Rockies, Cascades, and Sierras, where they no longer persist. Part 
of the explanation seems associated with the northward march of 
climate since the Pleistocene and part to isolation of small remnants, 
a factor related to the climatic shift. 

Barriers. Two opposite physical geographic features influence 
bird distribution. Barriers are features that prevent range extension, 
and spread-ways are the physical features that aid range extension. 
Tangible barriers appear on every hand. Expanses of water form a 





Fig. 10-2. Tertiary water gaps have prevented free passage of birds 
between North and South America. (A) Tehuantepec gap (late Miocene 
to middle Pliocene}, (B) Nicaragnan gap (late Eocene to late Oligocene), 
(C) Panamanian gap (late Eocene? to late Oligocene), (D) Colombian 
gap (middle Eocene to late Miocene). (After Ernst Mayr, "History of the 
North American Bird Fauna;' Wilson Bulletin, 58(1946):3-4l.) 

very definite barrier to land birds and to many shore and marsh ones 
also; expanses of land, on the other hand, form very definite barriers 
to waterbirds, especially to marine ones. A water gap between North 
and South America during much of the geologic past has prevented 
free passage of birds (Fig. 10-2). Other gaps have existed throughout 
the world in the past even as now though "island-hoppers" may 
cross them. 

Grasslands serve as barriers to forest birds and forest regions to 
grassland forms. Even within the oceans, we find barriers no less 
active for some species. Birds of the cold Antarctic and Sub-Antarctic 


do not range far northward except along cold ocean currents, like 
the Humboldt, which takes the Penguin into the equatorial zone on 
the west side of South America. 

To these barriers must be added some intangible ones, in a sense 
ecological, often of very great importance, though not so readily 
visible as the tangible ones: zonal (temperature, daylight), humal 
(atmospheric humidity), and SJsoc'mtiomt (territory, food, breeding 
places, cover, special needs). In their northward distribution, soutji- 
ern birds may be limited by temperature (coolness); they meet north- 
ern birds pushing southward, also limited by temperature (warmth). 
Presumably, a bird can survive in a colder or hotter region than the 
one in which it can breed successfully, just as a planted tree may live 
where it cannot reproduce efficiently. In the distribution of the 
Chestnut-backed Chickadee, it has been shown that atmospheric hu- 
midity limits their distribution along the dry California Coast (Grin- 
nell, 1904). Altitude plays a part, for the temperature gradient drops 
about 3 F. for each 1,000 feet rise of altitude. 

But the associational barrier is easiest to demonstrate, though some- 
times it may be the result of a zonal or humal condition. The Bob- 
white seems limited northward by snow depth and snow prevalence. 
The Passenger Pigeon did not inhabit the West, probably because it 
needed hardwood that the West could not provide, and perhaps also 
because it nested in trees and could not spread across the prairie. The 
importance of suitable habitat can hardly be overestimated as an 
ecological need of birds. Those lacking suitable habitat or forced to 
establish territories in submarginal habitat have difficulty reproducing 
or even surviving. 

Some habitat limitations are as definite as a vast ocean barrier, and 
a representative series of examples may not be amiss. The singing 
ground of a Woodcock limits it to habitat having such places. The 
absence of perches effectively stops the Tree Pipit, though not the . 
Meadow Pipit. The distribution of the Oregon Jay, Rosy Finch, 
Kingfisher, and Meadowlark is reported to be limited in California 
by temperature (Grinnell, 1917). Deep snow covering its food 
rather than cold is said to fix the northern limit of the Carolina Wren, 
and the effect of summer heat determines the distribution of the 
White Pelican in Texas (Griscom, 1945). The Everglade Kite (page 
426) does not range beyond the limits of the snail Pomacea (Ampul- 
larius) upon which it feeds (Symposium, 1945). The lack of water 
restricts Valley Quail in the desert. Peregrine Falcons must have 
cliffs on which to nest, and Burrowing Owls avoid hard soils. A 
field ornithologist can readily call to mind any number of examples, 
and a great many have been published (e.g., Armstrong, 1947, 1950). 


Pathways of Spread. Even though birds use the air freely, they 
require land connections for spreading outward from a center of ori- 
gin. Except for island invasions, birds seem to spread by slow, 
amoeba-like occupation of additional range. Spread-ways may be 
great land bridges across which birds may pass, such as between Asia 
and Alaska or North and South America. They may also be local 
crossings. The Alaskan land bridge, when connected, helped suc- 
cessive waves of immigrants in both directions. Subsequent breaking 
of the connection gave the needed isolation for the respective popula- 
tions on each continent to diverge before a reconnection of the land 
brought later invasions. Successive waves thus have been traced by 
systematists in the bird life of today. 

The Central American land connection between North and South 
America has existed in its present form only in recent times. For most 
of geologic time, only a series of islands lay between the continents, 
which provided "stepping stones" for island-hopping invaders, though 
not for others (Fig. 10-2). No land bridge is known between South 
America and the rest of the world in avian times, so that all land birds 
represented in both South America and elsewhere have had to pass 
through North America. 

It is axiomatic, however, that ecological conditions must favor a 
species for it to live in an area, and passage across a land bridge seems 
to offer no exception. Birds of the coniferous forest could hardly be 
expected to spread over a land bridge of tundra. Birds of the dry in- 
teriors of both Asia and America are separated now and probably have 
been for much of geologic time. 

But barriers and spread-ways are readily apparent within land 
masses. No fewer than nine species have invaded the desert region 
as riparian*nvifauna along the Colorado River in the Southwest. A 
great forest belt sweeps across northern North America to thrust 
many eastern birds northwest as far as Alaska, actually west of some 
of their western relatives farther south. The Myrtle Warbler, Yel- 
low-shafted Flicker, and Spruce Grouse reach places in Alaska actu- 
ally west of some Audubon Warblers, Red-shafted Flickers, and 
Franklin Grouse farther south. 

During more ancient times, a continuous environment seems a 
necessary postulate to account for birds of high latitudes now isolated 
from others of their kind. The Black-throated Green Warbler, Slate- 
colored Junco, and Red Crossbill of the southern Appalachians needed 
some such connection. The American Pipit, White-tailed Ptarmigan, 
Blue Grouse, and Clark Nutcracker live on isolated ranges and peaks 
of the Rockies, Sierras, or Wallowas, to many of which they probably 
spread by now nonexistent connections. 



Several birds appear fo be spreading northward now (perhaps but 
temporarily), and presumably the process as we see it here differs 
little from that of the past. The Cardinal, Mockingbird, Tufted Tit- 
mouse, Mouse Finch, Carolina Wren, and Red-bellied Woodpecker 
of America; and the Rook, Blue Tit, Goldfinch, Firecrest, Gray 
Wagtail, and Lapwing of Europe show this northward trend (Fig. 


Fig. 10 3. Extension of Lapwing range in Finland. (After Olavi 
Kalela, "Changes in Geographic Ranges in the Avifauna of Northern and 
Central Europe in Relation to Recent Changes in Climate" Bird-Banding, 

But it must not be assumed that birds are always expanding their 
ranges or that it is always northward. The Great Black-backed Gull 
has spread southward along the North American Atlantic coast. 
The \Vestern Kingbird, Lark Sparrow, and Lark Bunting have spread 
eastward in the American interior, whereas the Blue Jay, Baltimore 
Oriole, and Red-headed Woodpecker have gone westward. The 
range of the Dickcissel has receded from the Northeast (Fig. 10-1), 
while in Europe the Golden Oriole, Linnet, Bearded Tit, and Rock 
Sparrow have done somewhat the same thing (Kalela, 1949). 


Accidental Spread. Accidental (irregular) distribution of birds 
occurs and theoretically could assist the spread of birds, though from 
a practical standpoint, the chief accidentals are strong fliers and birds 
in the fall of their first year. Land birds appear as accidentals out of 
their ranges mostly in migration seasons, but even the most sedentary 
birds may be transported or wander great distances. Many sea birds 
appear in places far distant from their normal range and sometimes far 
inland in the wake of storms. Tropical birds especially are prone to 
be moved by tropical storms that carry them northward. In general, 
given time enough in any area, almost any bird of nearby or not-too- 
distant regions will appear sooner or later (it has even been suggested 
that "state lists" will be the same as the A.O.U. check list at some dis- 
tant, future date); 


Biogeographic Divisions of the World. The world has been di- 
vided up many ways to show distributional principles and relation- 
ships. Though each system has many points in its favor, none follows 
very faithfully the facts of bird distribution. In a sense this is as it 
should be, for the mobility of birds makes it possible for them to pass 
across many barriers to other animals and to be rather independent of 
positional restrictions. Their feathers and warm-bloodedness make 
them rather free of many climatic problems. Yet for descriptive pur- 
poses, various segments of the several distributional systems are vari- 
ously useful in bird work. 

The orthodox and long-standing system of Zoogeographic Realms 
suggested by Wallace divides the world as follows: 

1. Holarctic (North America, Eurasia, and North Africa), Pale arctic (Old 
World north of Sahara and Himalayas), Nearctic (New World north 
of southern Mexico). 

2. Ethiopian (Africa south of the Sahara) 
* 3. Oriental (Southeast Asia) 

; 4. Neotropical (New World from southern Mexico southward) 

t 5. Australian (Australia and adjacent islands) 


The Zoogeographic Realm system with its various modifications 
has many commendable features. Neotropical, Nearctic, Palearctic, 
and Holarctic have been used already in this book because they con- 
vey well an idea of the regions involved (Fig. 10-4). 

The famed Life Zone concept of Merriam divides North America 
into transcontinental latitudinal zones based upon temperature sum- 


mations supplemented by subdivisions based upon humidity. The 
principal zones are: 

Boreal Region 

Arctic Zone 

Hudsonian Zone 

Canadian Zone 
Austral Region 

Transition Zone (Alleghenian, Transition) 

Upper Austral Zone (Carolinian, Upper Sonoran) 

Lower Austral Zone (Austroriparian, Lower Sonoran) 
Tropical Region 

Tropical Zone 

Bigyjgj are still another representation of plant and animal distri- 
bution. (Akhough by definition bionics include animal life, they are 
essentially based upon plants.) The principal characteristics of the 
biome system are a series of large regions based upon the major fea- 
tures of plant distribution, which in the Holarctic world give Tundra, 
Coniferous Forest, Deciduous Forest, and Grassland biomcs. These 
are climax areas meaning permanent areas determined by climate, 
filter-grading areas are called gggj^gggf; and there may be subclimaxcs 
also (Pitelka, 1941). The biomes and ecotones of North America are: 

Tundra Biome 

Tundra Coniferous Forest ecotonc 
Coniferous Forest Biome 

Coniferous Forest Grassland ecotone 
Deciduous Forest Biome 

Deciduous Forest Grassland ecotone 

Birds cross all such boundaries the American Robin alone can be 
found breeding in every biome and nearly all ecotones. A map of 
biomes and life zones will be found in Aiidubon Bird Guide (Pough, 

Biotic provinces are areas rather similar in animal life, plant life, 
topography, and climate (Dice, 1943). They are natural regions 
rather readily visible to an experienced field ecologist and geographer. 
Biotic provinces are smaller than either biomes or life zones; some 
birds may cross them while others may be restricted to a single biotic 

The governing point in bird distribution seems to be the actual con- 
dition of the habitat the life form not the major feature of the 
landscape. A Horned Lark uses grassland or its equivalent, whether 
in a prairie, a tundra, or forest region. In the same way, trees are 
essential to the Downy Woodpecker; these may be groves, streamside 
trees, or actual forest. Tall trees are not needed by the Downy; 
neither are closed forests. Probably bird students will find vegetation 



types (page 204), such as "white pine type," "poa-fescue type," 
"sagebrush type," "red oak type," or "dune margins," the most useful 
means of indicating the kind of biogeographic unit wherein particular 
birds may be found. These may be named on the spot when published 
sources are inadequate (see also Chapter 11). 

Influence of Climate. As has been pointed out in the previous 
section, birds occupy the habitat suited to them wherever it may be 
found within the general limitation of climate. But the general distri- 
bution of the plants that supply the bird with its food and shelter 
depends upon climate. 

It is entirely clear that grassland birds live wherever suitable grass 
conditions may be found. But the great grasslands of the world all 
follow a general rule: they lie on the outer borders of the drylands 
where rainfall effectivity is low. The general rule for drylands is 
that they lie in rain shadows of mountains and on the western sides 
of continents, bending inward and poleward from about 20-30 




u) 45 

10 20 30 40 50 60 70 

10 20 30 40 50 60 70 80 

Fig. 10 5. Left. Climo graph of the Skylark in its optimum range (a) 
compared to Brooklyn, New York (b), and San Jose, California (c). 
Right. Climograph for the Kirtland Warbler breeding range (a) com- 
pared with the winter range in the Bahama Islands (b). The figures indi- 
cate the months. (After Arthur C. Tivomey, ll Climo graphic Study of 
Certain Introduced and Migratory Birds" Ecology, 11 (19 3 6): 122-1 32.) 



Fig. 10-6. The 'whiter distribution of species. Data from the Christmas 
Censuses, 1900-1939. (By permission pom Practice of Wildlife Conserva- 
tion, by Leonard W. Wing, p. 299. Copyright, 1951, John Wiley & 
Sons, Inc., 1951.) 

Latitude. Grasslands may be tropical savannas, short-grass prairies 
(stejpfes), or tall- grass prairies. In a similar general way, conifer 
forests are found where moisture is adequate and temperatures cool. 
(The American southern pines represent a special case of an edaphic 
nature.) Deciduous forests exist in humid and mid-latitudes; high 
temperature and rainfall in the Tropics make rain forests. Cold tem- 
peratures in the North result in tundra and at high altitudes mountain 
tundra and mountain meadow. Along foggy coasts may occur a special 
moisture-resultant flora. A cloud forest may exist on tropical moun- 


tain ranges where cloud moisture substitutes for rain. If rainfall is 
deficient, a desert results. Characteristic birds will be found in each 
of these appropriate areas. 

Sometimes climate is very effective directly or in combination 
with other environmental factors in limiting bird life. The long cold 
nights of the North limit the wintering of some birds, even though 
their needs otherwise seem supplied just as well as farther south. The 
food supply and other ecological needs for the American Robin 
seem adequate across the southern states in summer, but the tempera- 
ture appears to be too hot. A severe winter can reduce the Bob-white 
populations markedly in the Lake States. Humidity in the Coast 
Range is said to control the Gray Jay, even as it does the plant life 
(Grinnell, 1917). 

It is true that many birds can survive beyond the limits of their 
natural range, even in winter; birds in zoos and captivity have shown 
this to be true/'But clinwgraphs indicate that only in climates similar 
to the optimum* are birds likely to succeed in the wild away from their 
native range.) Climographic studies have demonstrated for some mi- 
gratory birds that temperature and precipitation in the winter fall 
rather significantly close to the breeding optimum (Fig. 10-5). More 
attention needs to be given to the relative climate of winter and sum- 
mer ranges of migratory birds before we can deal satisfactorily with 
this important matter. Importation of foreign birds has failed where 
the climate was unsuitable. 

The winter distribution of birds by species shows a northward 
thrust along the coasts of North America and southward in the in- 
terior (Fig. 10-6). This is primarily a reflection of the climate, but 
the influence of the climate is through the winter habitat itself as 
well as the temperature factor. 

Influence of Vegetation. The kind and quality of vegetation 
plays a most important role in bird distribution, and many parts of this 
book deal with it. One would not expect to find Meadowlarks in a 
hardwood forest, nor a Pileated Woodpecker out on the prairie. The 
Red-headed Woodpecker prefers oak trees, but the Three-toed 
Woodpeckers prefer conifers. Marsh Harriers may be found in large 
grass or grasslike areas if the vegetation is of the right height. 

In dealing with bird life in the field, people unconsciously recog- 
nize the great importance of vegetation. To call a bird one of the 
shore, brush, or fields in a way distinguishes it on the basis of vege- 
tation selection. Even within a single vegetation type there occur 
many differences of choice; one species may prefer the treetops, an- 
other the low limbs, and still another the forest floor. All correctly 
may be called "forest birds," but each occupies its own stratum. 


Influence of Topography. The influence of topography appears 
most in the mountain regions, especially pronounced in such moun- 
tains as those of South America and western North America. The 
rise of but a thousand feet in altitude is roughly equivalent to a pole- 
ward movement of 3 degrees of latitude (Fig. 10-7). Some birds are 
associated with high altitudes, others with intermediate altitudes, and 
still others with the lowlands. The vegetation influence of topography 
is marked, and probably much of the topographic influence acts 
through vegetation. The mountain influence is more spectacular in 
equatorial or dry regions than in humid or high latitudes (Fig. 10-8). 


Fig. 10-7. Comparison of the latitudinal and altititdinal zones or asso- 
ciations of plants providing the environments for animals. 

In addition to mountain topography, one must not overlook ordi- 
nary lowland topography river valleys, hills, plains, and even ditch 
banks. Changes of topography are found wherever land varies from 
a flat plain. Even slight variations may change vegetation and bird 
life. A river bluff means home sites to Kingfishers, Bank Swallows, 
and Sand Martins. Often the microclimates of topography are signifi- 
cant in matters of bird life. Cliff Swallows may avoid hot, exposed 
rocks in one particular area but occupy cool, protected ones around a 
bend. A south-facing slope ecologically may be scores or even hun- 
dreds of miles removed from a north-facing slope which is geographi- 
cally distant only a few hundred rods. 

River valleys and mountain tops have opposite effects. In North 
America, southern birds work northward up river valleys, while 
northern birds persist southward along the mountain tops. Canyon 
Wrens, Chats and other southern birds, for example, push far up the 
Snake River where the adjoining hills are the home of more northern 
species. In the Palouse Country and other parts of the West, one 



finds the Eastern Kingbird outnumbering the Western Kingbird in 
the hill country, but a drop of one or two thousand feet into a valley 
finds the reverse the case. The eastern bird favors moister hill coun- 
try, the western one the drier lowland. In parts of the Cascade foot- 
hills, the Mountain Bluebird may be found at the upper end of a hill 
pasture and the Western Bluebird at the lower end. 

Intraspecies Distribution. Certain "rules" of general application 
have been elaborated and may well be listed at this point. 

1. Warm-blooded animals tend to become larger in the cooler and smaller 
in the warmer parts of the species range (Bergmann Rule, Table 10 1). 

Table 10*1 
Weight Variations (Grams) Between Cool and Warm Regions * 

Species Cooler Region Warmer Region 




American Sparrow Falcon 



Mourning Dove 



Screech Owl 



Hairy Woodpecker 



Downy Woodpecker 



Horned Lark 



American Crow 



Blue Jay 



Black-capped Chickadee 



Tufted Titmouse 



Bewick Wren 



Brown Thrasher 



Pine Warbler 






Bronzed Crackle 






Pine Grosbeak 



Fox Sparrow 



(Average difference, 


* Bird weights are relatively little known, but these are probably indicative. 

Thus the Bob-whites of the South weigh about a fourth less than those 
of the North (Fig. 10-9A). 

2. Warm-blooded animals tend to reduce projecting body parts in the 
colder parts of the range (Allen Rule). This is not well shown by birds 
except for such things as reduction in bill length (Fig. 10*9B), though 
some birds may have longer and more powerful bills for feeding on 
frozen ground, such as the Chough of the Himalayas (Hingston, 1926). 

3. Birds of humid regions are darker than those of dry regions (Gloger 
Rule) (Fig. 10-9C). 

4. Birds of cooler regions lay more eggs per set than those of warmer 

5. Wings tend to elongate in mountains and cold regions. 


6. The races in cooler regions are more migratory than those of warmer 

7. Insular races have longer bills than others. 

8. Birds of cooler parts of the range have longer alimentary tracts; more 
migratory ones have shorter cloacas (and possibly shorter alimentary 

9. The more migratory birds lay eggs of greater length-breadth ratio 
(Averill Rule, page 104). 

Fig. 1 0*9. (A) The Bob-white of the South (left) weighs about a 
fourth less than that of the North (right) in keeping with the Bergmann 
Rule. (B) The bill of the Pine Grosbeak decreases northward in keeping 
with the Allen Rule, (a) Middle Rockies, (b) Canadian Rockies, (c) 
Alaskan interior. (C) Humid Region Song Sparrows (left) are darker 
than desert ones (Gloger Rule). 

Adaptability to Changed Environment. Questions of interpreta- 
tion arise over the adaptability of birds to changed environment. 
Adaptation, implies evolutionary processes, and adaptability to new 
environment really represents only substitutions of one set of condi- 
tions fo7"pfRefs 'within "the instinctive environmental pattern of choice. 



By adaptability to changed environment we should understand substi- 
tution ability and tolerance for changed conditions. 

No clear-cut principles can be drawn as yet from study of environ- 
mental changes and birds, but a few generalities have merit. Like 
other organisms, birds have a threshold of habitat suitability along 
with an upper limit of exclusion. The optimum habitat rests between 
(though not necessarily equidistant from) the extremes. The ecologi- 

Ruffed Grouse 

Sharp-tailed Grouse 


ne chicken 



(Grasses, shrubs, 



(Grasses, many 

shrubs, many 

open woodlands) 


(Young forest, 
heavy brush, 

few grasslands) 

(Dense, usually 

old, often 
climax stands) 

A. Lower habitat threshold. B. Optimum habitat. 
C. Zone of partial or complete exclusion by succession. 

Fig. 10*10. The overlapping requirements of the several Wisconsin 
Grouse ilhistrate the succession of species with change of vegetation in 
accord with ecological succession. (After Wallace B. Grange, Wisconsin 
Grouse Problems, p. 238. Madison, Wis.: Wisconsin Conservation De- 
partment, 1949.) 

cal distribution of Grouse in relation to plant succession illustrates 
these points very clearly (Fig. 10-10). So long as conditions are 
suitable, the birds will continue to occupy an area, though they thrive 
best in optimum habitat. 

In the course of human occupation of the land, changes usually 
mean retrogression on the ecological scale, such as are caused by the 
cutting of timber or plowing of the prairie. But succession itself 
advances the habitat. The most drastic bird upsets occur in the 
"down-grading" phase, but the longer-lived ones usually occur with 
succession advance. American birds well known for their tolerance 
of changed conditions include the Crow, Song Sparrow, Robin, 
Chipping Sparrow, Yellow Warbler,' House Finch, Bob-white, and 



Blue Jay. Those well known for their intolerance include the Raven, 
Kirtland Warbler, Ivory-billed Woodpecker, Pileated Woodpecker, 
and Spruce Grouse. 

The spectacular success of the European House Sparrow and 
Starling in America illustrates the substituting ability of a species (Fig. 
10-11). Just what their ecological adjustments were under native 
conditions is not clear, for both have been closely associated with 

Fig. 10' 1 1. The House Sparrow spread faster in America following 
its importation than did the Starling. (Adapted from Leonard W. Wing, 
"The Spread of the Starling and English Sparrow;' Auk, 60(1943):76.) 

man's artificial environment during ornithological history. Both per- 
haps nested in holes and crevices or perhaps occasionally built a nest 
in trees. House Sparrows in the South, for example, build their nests 
in trees far oftener than those in the North and also use buildings less 
often. They use palms freely. Perhaps reactions to air temperature 
account for their use of more exposed sites for nesting in the South. 
Table 10-2. shows some substitutions by birds of man-made structures 
for natural ones. 

Bird students generally assume that birds of the brush are more 
tolerant than others. So many common birds of the garden and farm 
have increased in abundance since settlement that this appears logical, 


Table 10*2 
Substitution of Man-Made Structures (or Natural Ones. 


Original Site of Nest 


Wood Duck 

Tree cavities, 


Nest boxes 

American Sparrow Falcon 

Tree cavities, 


Nest boxes 

Screech Owl 

Tree cavities, 


Nest boxes 

Purple Martin 

Tree holes 

Nest boxes 

Barn Swallow 

Caves, ledges 


Cliff Swallow 


Walls, bridges 

Tree Swallow 

Tree holes 

Nest boxes 

Violet Green Swallow 

Tree holes 

Nest boxes 

Eastern Phoebe 

Cliffs, caves 

Eaves, rafters, bridges 

Chimney Swift 

Hollow trees 


American Robin 


Buildings, bridges 

Crested Flycatcher 

Tree holes 

Nest boxes 




House Wren 

Tree holes 

Nest boxes 

though it may not necessarily be so. Birds of earlier ecological stages 
have increased because more of this habitat is available. But wherever 
other habitat has developed, bird life has increased to fill it. The 
Horned Lark of the prairie has expanded its range, even in forest 
regions, wherever grass has replaced other vegetation. The Blue Jay 
of the forest has spread out into the plains where streamside ribbons 
and farmyard groves of timber provide the needed habitat. Adapta- 
bility to changed conditions in historic times seems to reflect tolerance 
for varying degrees of habitat suitability and a somewhat elastic ca- 
pacity within the limits imposed by instinct. 


Size of Range. The size of range occupied by a species seems de- 
termined by a great many conditions of the bird and environment that 
together form the range complex. Chief among those of the bird are 
its instinctive habitat requirements, habitat tolerance, and adjustment 
to climate. Those of environment include vegetation, climate, and 
topography, as well as many others. But the actual determinant 
within the complex may be rather restricted. Small things within an 
area often determine use made of the habitat by birds or their survival 
beyond the general pattern. Limb density just below the forest 
crown, for example, is a critical factor limiting its use by Least 
Flycatchers; few Flycatchers used habitat having less than about 30 
per cent openness (Breckenridge, 1956). Within broad areas of single 
vegetation type, niches for certain species may be small and limited to 


catastrophic areas in the forest, such as blowdowns, fire burns, eroded 
spots, or landslides. 

The wide-ranging Osprey occurs throughout the world, and its 
range may be measured by continents. No other nonoceanic bird 
(except possibly the Barn Owl) touches practically all lands of the 
earth. In the case of oceanic birds, the great uniformity of oceanic 
environments provides the space for great ranges. Yet oceanic spe- 
cies may be confined to single ocean areas or zones of latitude. % 

The complex character of land environment operates against a 
single species spreading over much of the world while maintaining 
its species identity. The Raven of the Holarctic has done so with 
notable success. The Horned Owl of the New World ranges from 
the Arctic to the tip of South America, the greatest range among 
wholly New World species. It illustrates what often happens in wide- 
ranging land birds they tend to subdivide. 

Restricted ranges characterize many island birds and sometimes 
continental ones also. The Kirtland Warbler, for example, occupies 
but a few thousand square miles of breeding range in Michigan and 
the Golden-checked Warbler no more in the Edwards Plateau of 
Texas. Even more restricted is the range of the Cape Sable Seaside 
Sparrow, which inhabits only about two thousand acres of coastal 
prairie at Cape Sable, Florida (but it may be only an isolated pocket 
of birds belonging to a wider-ranging species). Other examples are 
the Ross Goose, Labrador Duck, Whooping Crane, Bristle-thighed 
Curlew, and Bachman Warbler. They may be relict species that sur- 
vive where competition is least severe, as on islands or on continents 
by becoming more and more specialized. A relict species is one whose 
numbers or range or both have undergone drastic reduction ( Amadon, 

v Continuity of Range. Generally speaking, the over-all range of 
birds" forms a continuous one with but scattered pockets beyond the 
main limits. This is true especially perhaps of expanding ranges and 
those of more recent invaders to a land mass. The Mockingbird of 
southern United States occupies a continuous range broken only by 
local variations in its preferred habitat. But as mentioned elsewhere, 
the range of most birds consists of occupied habitat in a matrix of 
unused space. The Magpie of the West has spread eastward, but not 
by leap-frogging over suitable habitat nor have the Horned Lark, 
Song Sparrow, Black-capped Chickadee, or a host of others whose 
ranges have increased. 

Discontinuous Range. It is often easier to illustrate discontinuity 
of range than continuity, for the gap between calls attention to col- 


onies of birds distant from the main body. Just as continuous range 
may mark an expanding or static species, discontinuous range seems 
often to be the mark of a species whose range has shrunk or shifted, 
sometimes the result of a major geological event. The most recent 
of these, the Ice Age, has left its mark on many discontinuous ranges. 
The Red-breasted Nuthatch, for example, occupies much of the 
northern coniferous forest of North America. Two remnants live 
in the Old World, one atop the mountains of Corsica in the Medi- 
terranean Sea and a second in Eastern Siberia and China. All are 
distant from each other. Since these birds have low flight powers, we 
can justifiably conclude that they are remnants living in separated parts 
of a former wide range, rather than in advance pockets of recent 
expansion. The probable explanation lies in a formerly widespread 
Holarctic distribution broken up by events of glacial times. More 
recent geological events may have been involved also. 

Influence of Mobility on Range Occupation. The very great 
power of flight does not relieve birds from the limiting actions of 
distance it only sets the limits farther away. Nesting, resting, feeding, 
and escape cover must be at hand for a bird of high as well as of low 
flight powers. A Mockingbird requires trees in open grass areas, all 
within a few yards or rods, but the habitat needs of a Golden Eagle 
may be separated by miles. A desert bird requiring water daily 
would be limited by its travel distance, as would also any bird in 
finding its daily food. But distinguishing flight limits as a control 
from actual instinctive aversion makes any interpretation difficult. 

An ingenious calculator concluded that because the Chimney Swift 
seldom perches from early morning to late at night (in good weather 
at least), a banded one known to have lived at least 9 years flew 
1,350,000 miles in the nine years of living and nine trips to South 
America and back to the United States. For each year of life beyond 
nine, 150,000 more miles would be added. For a bird that remains 
aloft for hours at a time, the interspersed nature of its living needs 
might not be so important a control as for a bird of less flight range. 
In any event, makers of precision equipment might well envy the 
lasting qualities of the bird mechanism. 

Distribution of Closely Related Species. The closer the rela- 
tionship of species, the closer ecologically and geographically should 
we expect to find their ranges, but historical events may alter this 

The ranges of tf//0po.species (geographic complements) may be 
expected to form a continuous range. One species declines in num- 
bers as it is replaced by the other and the combined density in the 







Fig. 10' 12. The winter ranges of the allopatric Flickers illustrate geo- 
graphic complementing, the one species declining in numbers as the other 
increases. The figures are the average birds-per-hour of the Christmas 
Censuses, 1 900-1 939. A dotted connection between the respective isoplcth 
line indicates combined abundance. 

zone of overlap remains equivalent to the separate abundance in the 
respective "areas, as shown by winter data of the Red-shafted and 
Yellow-shafted Flicker (Fig. 10-12). The Mallard and Black Ducks 
show this also (Fig. 10-13). 

The ranges of sywpatric species, like the Hairy and Downy Wood- 
peckers, overlap rather generally, but the larger member of the pair 
tends to have the more northerly distribution, at least in winter (Fig. 
10-14). Exceptions may occur, however, as in the Cooper and Sharp- 
shinned Hawks. The smaller Sharp-shinned nests farther north, but 
the larger Cooper Hawk tends to winter farther north in greater 
numbers and biomass. 

Origin of North American Bird Life. Though the past history of 
bird life does not furnish the evidence for detailed tracing of origin, 
development, and spread (as has been done with some larger mam- 
mals), a general pattern of origin for North American families has 
been proposed (Mayr, 1946). The largest contributors of species are 
North America itself and the Old World (Fig. 10-15). 



Fig. 10-13. The ratio Hues in die ale change in proportions of Mallard 
and Black Ducks as reported in Christmas Censuses. The Mallard out- 
numbers the Black Duck in the West. (From Leonard W. Wing, "Rela- 
tive Distribution of Mallard and Black Duck in Winter" Auk, 60(1943): 
439, and Christmas Census data, 1900-1939.) 

North America shares with South America a number of families 
that have not spread into the Old World (except incidentally on the 
Siberian shore) though they are expanding their ranges now. Ex- 
amples of these are the New World Warblers (Parulidac), Vireos 
(Virconidae), Flycatchers (Tyrannidae), Tanagers (Thraupidae), 
Hummingbirds (Trochilidae), and Icterids (Icteridac). But some 
seemingly vigorous Old World birds (e.g. Hoopoes, Pittas, Rollers, 
Sunbirds) have not reached the New World, at least so far as living 
birds of today are concerned. 

The nonoceanic birds of North America represent several areas 
of origin which have been listed as the Pantropical, Old World, North 
American, and South American, with perhaps a Pan-American one 
(Mayr, 1946). 
Examples of each follow: 






Fig. 10 14. The 'winter ranges of sympatric Hairy and Downy Wood- 
peckers show no geographic complementing. The numbers shown for 
the Plains Country reflect concentration of birds hi river bottoms. 
(Christmas Census data, 1900-1939.) 

Fig. 10*15. Diagram of the faunal elements of North America. (After 
Ernst Mayr, "History of the North American Bird Fauna" Wilson Bul- 
letin, 58(1946)3-41.) 
































Enough is known of families (or their groups have diverged enough 
in some cases) that origin of subgroups appears determinable. In the 
Finch family (Fringillidae), the subfamily Carduclinae appears to 
have immigrated from the Old World, Richmondeninae came from 
South America, and Emberizinae developed in North America. The 
Kinglets of the family Sylviidae came to America from the Old 
World, but the Gnatcatchers arose locally. The Quails (Odontophor- 
inae) arose in America, but the rest of the Phasianidae, as well as the 
family itself, arose in the Old World. 

As would be expected, the Old World element appears most fre- 
quently in the North American boreal zone (geographically nearest 
to the.Old World land mass) and the South American element in the 
austral zone. The North American element appears most frequently 
in the more isolated middle latitude and especially in the interior 
prairie region (Table 10-3). The interior prairie region seems to 
have had few floral connections with comparable vegetation zones. 
(The marine influence on climate in the North Pacific would have 
precluded a connection northwestward, and both dryness and the 
wet tropics would have done likewise southward.) 


Table 10-3 

Analysis by Geographic Origin of the Breeding Passerine Species of Several 
North American Areas 


of South 

of North 

of Old 

Yakutat Bay (Southeast Alaska) 



58 * 





Nipissing Area (Southern Ontario) . 
New Jersey 








Sonora, Mexico 




Source: Ernst Mayr, "History of the North American Bird Fauna," Wilson Bul- 
letin, 58 (1946): 3-41. 

Within the North American continent, western birds drift and 
wander eastward far more often and more successfully than the con- 
verse. The general westerly winds of mid-latitudes may be involved. 
Representatives of Old World families have penetrated the American 
Tropics. It may well be that birds of cooler regions become adapted 
more easily to warm climates than the opposite. But because the 
climate occurring at the Bering land bridge would govern any transfer 
of bird life, northern birds would always have the advantage. The 
past climate of the North Polar region has varied (Berry, 1930). 
Only by progressive adaptation and spread or only when climatic 
belts move bodily northward would southern birds be in a favorable 
position to cross. An arid zone across northern Mexico and south- 
western United States has probably acted as a barrier from very 
ancient times, as has also a more humid zone nearer the Alaska land 

The invasion route across the Alaska land bridge is on the western 
side of North America. Like all immigrants, the new arrivals settled 
down in the nearest suitable land. Their descendants have slowly 
spread eastward to occupy ranges variously eastward from the western 
side where first their ancestors landed. There seems to be no reason 
to assume that the process has ended, either for those having already 
gone across the land bridge or for others that might do so. The path 
of spread between the Old and New Worlds hence accounts for the 
greater similarity between the bird life of western Europe and western 
North America than between western Europe and eastern North 
America, several thousand miles closer. The Nutcrackers, Waxwings 
(Bohemian), Magpies, Dippers, and many others of western North 
America are found also on the Eurasian continent. 



* ARMSTRONG, EDWARD A., Bird Display and Behavior. London: Lindsay Drummond, 

Ltd., 1947. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
DICE, LEE R., Biotic Provinces of North America. Ann Arbor, Mich.: University of 

Michigan Press, 1943. 
*GRISCOM, LUDLOW, Modern Bird Study. Cambridge, Mass.: Harvard University Press, 

HESSE, RICHARD, W. C. ALLEE, and KARL P. SCHMIDT, Ecological Animal Geography. 

New York: John Wiley & Sons, Inc., 1951. 
LACK, DAVID, "Ecological Aspects of Species-Formation in Passerine Birds," Ibis, 

86(1944) :260-286. 
MAYR, ERNST, "History of the North American Bird Fauna," Wilson Bulletin, 

58 (1946): 3-41. 
MILLER, ALDKN H., "Habitat Selection Among Higher Vertebrates and Its Relation 

to Intraspecics Variation," American Naturalist, 76 (1942): 25-35. 


Ecological Relations 

of Birds 

While there are many definitions of ecology, it seems most satis- 
factory still to return to the early one that ecology is the study of 
the relation of an organism to its environment. Hence, most activities 
of living birds come under the term ecological relations in the broadest 
interpretation of the word. It seems best at this point, however, to 
consider only a few of these relationships. 

The success or failure of an individual bird (and the species to 
which it belongs) lies in the reactions of its body mechanism physi- 
cal, physiological, and otherwise to the biological hammering and 
cushioning of the environment. This goes on all day long and all 
night long; it continues in summer and in winter. It goes on during the 
fleeting instant when a bird passes across the observer's field of vision, 
just as it does during the rest of the bird's life, though unseen by an 
ornithologist as the bird lives out its life in the bush, the tree, the 
marsh, the shore. Some of this matters little to the bird, some of it 
matters greatly. But to students of nature and its science, it tells a 
story; that story we call ecology. 


Biological Energy. The source of all energy for the bird comes 
from the sun, and all birds obtain this energy second hand from the 
plants or even further removed if taken from other animal life (see 
Chapter 23). Plants form the biological portal through which energy 
enters the bird world. Plants, which are about 18 per cent efficient 
in using solar energy, by photosynthesis transfer energy from the sun 
to organic compounds through appropriate chemical action involving 



primarily carbon dioxide of the air, water of the soil, and soil 

The biological energy potential is highest in equatorial regions 
where the'greatest amount of solar energy reaches the earth. In mid- 
latitudes, the energy usable in photosynthesis amounts to a flow of 
about 150 horsepower per acre. The most important earth influence 
determining how great an expression the biological energy potential 
reaches is moisture, for only when they have sufficient moisture will 
plants utilize to their fullest the energy of the sun. The desert coast 
of South America lies under the full tropical sun, for example, but 
plants there do not extract from the sun the great quantities of bio- 
logical energy entrapped within a Michigan woodland or an Oregon 
forest. With about the same solar exposure, the coast of South Caro- 
lina has a greater total of plant and animal life than that of southern 
California. The reason, as stated above, is that areas with a goodly 
water supply extract more biological energy than dry ones. 

The entrapment of biological energy may vary also with other 
environmental factors. Along a coast where cold winds blow in from 
the sea, the low temperature does not permit plants to extract energy 
as they would at the same latitude with warmer air temperatures. The 
presence of fog and cloud screens out the sun's rays. A water habitat 
may produce more life than a land habitat because of water avail- 
ability, but local conditions of turbidity, temperature, acidity, or 
alkalinity may interfere. Iricias^aitim^ 

of sun's rays because of less atmospheric screening, but the cooler 
temperatures of higher altitudes offset these gams. 

Biological Energy Competition. It is axiomatic in thermody- 
namics that when two mechanisms compete for the same source of 
energy, the more efficient of the two prospers at the expense of the 
less efficient one. We should expect in nature that the more efficient 
organism has an advantage over a less efficient one. By that token, the 
environmental complex native to an area appears to be the most 
efficiently adapted for use of the biological energy of that area. In 
terms of ecology, it means that the climax is the most efficient stage. 
The principle assumes, which is a point of weakness, that a complete 
array of life has had access to the area and the survivors are the cur- 
rent victors in the struggle for use of the area's biological energy 


Ecological Succession. An area undisturbed and in adjustment 
with the climate ^termed. QjtJWAX* Adjustment to the climate means 
that plant and animal life live essentially in a state of equilibrium, the 


entire complex renewing itself as before. But when such an area is 
disturbed, it returns to the climax through successive recognizable 
and predictable stages. This process is ecological succession, each 
step a stage, and all steps together constitute a sere. Succession in 
water habitats forms a hydrosere, that of land a xerosere. (Weaver and 
Clements, 1938). An example of a hydrosere (freshwater) with some 
representative birds may be listed as: 

Submerged Stage 

Water less than 20 feet deep, Diving Ducks, Grebes, Coot 
Floating Stage 

Water 6 to 8 feet deep, Dabbling Ducks, Gallinulcs, Coot 
Reed-Swamp Stage 

Water 1 to 4 feet deep, Dabbling Ducks, Gallinules, Rails, Red-winged 

Sedge-Meadow Stage 

Water below but near surface (except in flood), Meadowlark, Savannah 

Sparrow, Bittern 
Grass Stage (in grassland region), Meadowlark, Lark Bunting 

Woodland Stage (in forest region), Wood Thrush, Blue Jay. 

A xerosere (dry land) example beginning with bare ground and 
some representative birds may be listed as: 

Crustose-Lichen Stage, no characteristic birds 

Foliose-Lichen Stage, no characteristic birds 

Moss Stage, no characteristic birds 

Herbaceous Stage, Vesper Sparrow, Meadowlark, Horned Lark 

Shrub Stage, Cardinal, Yellow Warbler, Chat 

Climax Forest, Acadian Flycatcher, Scarlet Tanager 

The speed of succession varies with regions. In low latitudes it 
is rapid, often passing from the bare ground to the forest stage in 10 
to 20 years. But near the Llvnt of Trees in the Arctic, the time neces- 
sary for comparable progress may be ten times that of mid-latitudes. 
A schedule in Tennessee perhaps would be as follows (Wing, 1940): 

Bare ground first year 
Weeds first to third year 
Forbes second to fifth year 
Grass fourth to sixth year 
Briar fifth to seventh year 
Brush sixth to twelfth year 
Forest ninth to eighteenth year 

Habitat. The environmental complex that satisfies the bird is the 
habitat, though we commonly think of habitat only as the vegetation 
type in which a bird lives. The term habitat in the animal world, it 



should be noted, has much the same meaning as site to the plant in 
the plant world. 

Within the environment exist combinations of habitat factors that 
provide the life-opportunity for each species. Each combination of 
factors forms a niche occupied by a species. In the^ook^QIffs^o'f 
Utah, tKree species of birds live' permanently in the pigmy conifer 
forest but avoid competition by differences in food habits and nest 
selection (Hardy, 1945). Five semipermanent species avoid this com- 
petition by using nest cavities of different sizes, by nocturnal habits, 
by different feeding habits, and by different spatial habits. In the 
competition of life, it seems axiomatic that no two species of birds can 
occupy the same niche at the same time any more than two objects can 
occupy the same space at the same time. 

The birds of any major habitat have developed differences in niche 
requirements, partly from behavior patterns, that avoid intersgecics 
competition and strife (Fig. 11-1). The New World Warblers 
(Parulidae) illustrate this very well in their selection of territories. 
Twelve species living together in the mixed conifer and deciduous 
forest region of eastern New York, an area severely disturbed by man, 
have separate niches determined in part by differences in the vegeta- 
tion used and in choice of nesting site (Table 11*1). Similar varia- 
tions in choice of vegetation occur in other groups of related birds or 
birds of the same general environment. 

Table 1 1 I 
Nest and vegetation selection by Warblers 


Vegetation Selected 

Nest Site 

Black and White . ... 

. . . Tree trunk 



Sunlit open conifer 



Low level conifer 

Conifer branches 

Black-throated Blue 

Shaded shrub 

Shaded brush 

Black-throated Green . 

Middle level conifer 



. . . Top level conifer 



. . . Sunlit shrubs 



Dry shaded ground 

Dry ground 

Louisiana AVater-thrush . . . 

\Vet shaded ground 

Moist ground 


. . . Wet sunlit ground 

Low vegetation 


. . . Moist shaded ground 

Moist ground 


Second growth hardwoods 

Low forks 

Source: In part after S. Charles Kendeigh, "Community Selection by Birds on the 
Helderberg Plateau of New York," Auk, 62(1945):418-436. 

\ Although the macroclimate of a region may remain much the same, 
J)irds may respond to differences of microclimate, a matter of which 
we know little. Other environmental factors are subject to variations 


in choice. Some birds prefer the low light intensities of shade and 
others the high light intensities of tree tops or the open. Even perches 
play a part in the choice of niche. 

Major Habitats. For purposes of simplicity, it is customary to list 
birds by their major habitats. The ten broad designations most com- 
monly used are forest, brush, grassland, desert, marsh, lake, stream, 
shore, ocean, and bare ground. But each of the ten may be subdivided 
into a large number of habitats. The forest for example, may be 
divided readily into conifer, deciduous, and broadleaf evergreen 
forests. The latter may be divided again into tropical, subtropical, and 
mid-latitude broadleaf forests. The grassland (open range) may be 
prairie, steppe, savanna, meadow, dune, and sometimes moor, tundra, 
or mountain tundra. Deserts may be of many kinds, such as chaparral, 
thorn thicket, cactus, shrub forest, sagebrush, salt flats, or "desert 

The ocean covers 72 per cent of the earth's surface yet it contains 
but a small fraction of the bird life. Even the birds able to live pelagic 
lives, like the Shearwaters, Petrels, and Albatrosses, must return to 
land for nesting. So far as marine bird life is concerned, its greatest 
abundance occurs where the largest quantity of available, supporting 
marine organisms are found (Murphy, 1936). 

The largest habitat in North America and perhaps Eurasia is the 
open range habitat, which embraces tundra, grassland, and much of the 
desert; it contains about 40 per cent of the land area in North Amer- 
ica. The second largest is the conifer forest habitat, with 30 per cent 
in North America; and the third is the deciduous forest habitat, with 
about 15 per cent. For the world as a whole, the major habitats from 
the natural vegetation standpoint may be estimated as in Table 11*2. 

Table 11*2 
Estimated Thousands of Square Miles of Major Bird Habitats in the World 





Forest . . 

21 500 


... 550 












.... 138,000 



Vegetation Zones. The bird student in the field may not often 
find any named systems described in Chapter 10 useful except perhaps 
as general or broad geographic designations for which purpose 
political, geographic, or climatic regions may be just as suitable and 



sometimes more so. But for major comparisons, biogeographical 
designations may serve a very useful purpose. It is clear that no 
system yet devised will serve adequately the needs of bird students, 
though such terms as Canadian or Coniferous Forest convey a definite 
meaning. If the nature of the systems is understood, however, one is 
about as useful as another for bird work. 

In the field, vegetation zones usually will be the most useful clas- 
sification for bird work. Such zones are really cover types, the scale 
of zonation being governed by fineness of the study. Vegetation zones 
suggested for the northern intermountain region (Daubenmire, 1946) 
serve as an example of altitudinal zones useful in bird work: 

Mountain Tundra 
Sedge-grass zone 

Conifer Forest 
Spruce-fir zone 
Arborvitae-hemlock zone 
Douglas fir zone 
Ponderosa pine zone 
Juniper-pinon zone 


Fescue-wheatgrass zone 
Whcatgrass-bluegrass zone 
Needlc-grass-gramma grass zone 

Semidesert and Desert 
Sagebrush-grass zone 

Table 11-3 
Vegetation Zones of Mexico 

Per Cent 
of Area 

Square Miles 
of Area 

Yearly Rainfall 
in Inches 

Boreal forest 


3 800 


Pine-Oak forest 


195 600 











.... 23.1 



Cloud forest . . . . 




Rain forest 




Tropical evergreen forest 








Tropical deciduous forest 




Thorn forest 




Arid tropical scrub . . 







(Average 38.6) 

Source: A. Starker Leopold, "Vegetation Zones of Mexico," Ecology, 31(1950): 


Vegetation zones may parallel climatic conditions, and the major 
vegetation zones of Mexico illustrate this rather well (Table 1 1 3). 

Life zones and biomes (Chapter 10) have the fundamental weak- 
ness of using the climax as their bases. Since man has interjected him- 
self into the landscape, so many areas have been so completely subju- 
gated to his action that climax classifications may have only academic 
or historical significance. Vegetation zones or types that indi- 
cate by their names the principal character of the type are useful. 
Thus, second-growth hickory, sagebrush-bunchgrass, and thorn forest 
mean something definite. Though not often to be recommended, 
terminology based upon the bird life of an area might be used if one 
so chooses. A Hermit Thrush zone, Meadowlark zone, or Yellow- 
throat zone would mean something to a bird student, though probably 
not much to others. 


Cover Type. The preceding sections have suggested the use of 
classlHcations tKat indicate the land as it exists today or that are based 
upon the land as it now appears. In a sense, these are all cover types, 
though we usually use the term for small areas, generally for local 
vegetational types. A farm or a forest may have a dozen cover types 
within its borders. A common method of indicating forest cover 
types, for example, is by means of tree species, size, and spacing. A 
cover map forms an excellent base on which to indicate bird terri- 
tories, movements, nests, or other field data. Cover types for a general 
midwestern American farm might include some of the several given 


Fallow fields Brush fence rows 

Weed fields Grass fence rows 

Ragweed Seeded pastures 

Hay field Grass pastures 

Orchards Marsh 

Abandoned fields Brush 

Beech woods Brush draws 

Oak woods Swamp hardwoods 

Hickory woods Brush pasture 

Interspersion of Cover. In a sense, birds require a variation in 
cover just as do other organisms, which in the breeding season usually 
involves a mixture of cover types or species. Cover formed of a 
single plant species usually supports far fewer birds than a mixture 



of several species; a single species cover type of all age and size classes 
has greater attraction than a uniform one. The mixing of plant species 
(interspersion) directly affects bird numbers and success. In addi- 
tion to this factor, the various requirements of the bird must be within 
reach as governed by its cruising distance. 

The Mockingbird of southern gardens and lawns illustrates the 
need for interspersed cover. Its needs include (1) grass areas such as 

Field Sparrow 
Yellow-billed Cuckoo 

Red-eyed Vireo 

Fig. II *2. Left. Territories of the Cardinal, Field Sparrow, and Yel- 
low-billed Cuckoo, all forest edge birds. Right. Territories of the Red- 
eyed Vireo, a forest interior bird. The dots indicate a 50-meter grid of a 
56-acre woods near Urbana, Illinois. (After Verna R. Johnston, "Breed- 
ing Birds of the Forest Edge m Illinois" Condor, 49(1941):45-53.) 

lawns on which to hunt food, (2) brush in which to nest, rest, hide, 
and feed, and (3) trees from which to sing and in which to roost. A 
150-acre area in which these needs were proportionately spaced could 
have a pair to the acre and perhaps even more. But if all the trees 
were concentrated in an 8-acre woodlot, all the brush in a 30-acre 
field, and the grass in a pasture, the 150-acre tract would have no 
Mockingbirds except for a few where the woodlot, brush, and 
pasture meet. 


The advantages of access to more than one kind of cover along 
the boundary where cover types meet show in the greater abundance 
of birds there. Some species will be found only in the forest interior, 
some in the interior and border, and some only along the border 
(Fig. 11*2), but they are usually most numerous along the edge. 
From this has developed the concept of edge effect, an edge being the 
meeting of two plant types. Edges exist in open fields where grass 
gives way to weeds, in lakes where the shallow-water Scirpi/s gives 
way to water lilies, and in the forest where pines give way to spruces. 
(Ecologists, be it noted, sometimes call an edge by the term ecotone.} 

Cover Volume. Forests tend to have stratification of bird life, so 
that several species may occupy the same ground surface but at dif- 
ferent levels. The volume of vegetation available to birds thus plays 
an important role in bird numbers. Ornithologists have still to de- 
termine the factors governing abundance of species and numbers in 
relation to volume. That it is not proportionate seems certain. One 
of the reasons for this rests in the fact that the energy reception by 
the forest concentrates in the canopy where light is greatest, just as 
it does in a brush patch. A forest of trees 100 feet high, especially 
the nonconifers, has about the same crown exposure to catch sunlight 
as a forest of 50-foot trees. The difference in entrapment of bio- 
logical energy lies largely in the space between the crown and the 
ground. In some forests this may be rather uniformly filled with 
shrubs and small trees, but in others there is little undergrowth. The 
light from the sun that filters down is but a part of that hitting the 
forest roof. 

Shelter and Roosts. The shelter needs of most birds may seem to 
need little discussion, they are so abundant; yet exact knowledge of 
even general shelter needs is far from adequate. As most shelter 
requirements will be covered in appropriate places, it may be unneces- 
sary to elaborate greatly upon the subject in this section. Shelter itself 
sometimes differs in connotation from escape cover in that a bird uses 
shelter for protection from the elements and escape cover for protec- 
tion against enemies. As has been indicated already, both are definite 
physical identities that must satisfy the instinctive make-up of the bird. 

Night roosting occurs in characteristic situations, all related to the 
bird's diurnal habits, but our knowledge of the subject is woefully 
deficient. Hole-nesting birds generally roost at night in holes, often 
the one used for nesting. Chickadees sometimes roost together, per- 
haps a whole flock in one hole if it is large enough. The Swallow 
habit of roosting among the reeds in marshes is well known. The 
Chimney Swift roosts in chimneys, sometimes by the thousands during 



migration. During the summer in parts of the West, Cliff Swallows 
spend the night in their nest-pots fastened to cliffs. In winter these 
same nest-pots may be used by Rosy Finches from the high country 
(Shaw, 1936). 

The average Passerine bird roosts in brush or in trees, usually 
hidden in the foliage or twigs. Most prefer thick branches or even 
vines when available. But some birds, such as Nighthawks, perch on 
the main branches (sometimes in flocks made up largely of males 

Fig. 1 1 3. The night-roosting Bewick Wren fluffs up its plumage to 
form a ball of feathers. (Photograph by Laidlaiu Williams.) 

during the summer). Owls often roost on a branch, sometimes close 
to the trunk. Birds like the Song Sparrow settle down in brush or 
thick marsh vegetation. The White-throated Sparrows and others of 
brush and hedgerows perch in the brush for the night. 

Ground birds, even Passerines, may spend the night on the open 
ground or in clumps of grass. Savannah Sparrows may settle on the 
bare ground and Meadowlarks in the grass. The Valley Quail roosts 
in low, heavily foliaged trees. The hours on the roost reported in 
California varied from 8 hours, 18 minutes on July 5 to 13 hours, 33 
minutes on November 26 and probably still more in December and 
January (Sunnier, 1935). The Ruffed Grouse spends the night in 
thick, wind-resistant clumps such as conifers or grape tangles, usually 


close to the main trunk. (In cold weather when the snow is deep, 
they may burrow under the snow for the night.) Ptarmigan may 
burrow under the snow or roost in depressions in the snow. Prairie 
Chickens and Sharp-tailed Grouse may fly several miles to favored 
dry marshes or other sites for the night. 

Most ducks and other waterbirds may spend the night in the open 
water, but some spend the night on shore. Shorebirds and many Gulls 
and Terns spend the night on the bars, beaches, and shore. 

The roosting bird fluffs up its feathers, which increases the thick- 
ness of the insulating air blanket. A Bewick Wren pulls its head in 
and fluffs up like a ball, with only its tail extending beyond the smooth 
contour of the feathers (Fig. 11-3). Some birds turn the head back 
into the scapular and back feathers. This may protect the eyes, 
nostrils, and thinlv covered face from cold in cold regions, but some 
birds of tropicarand subtropical regions do likewise (Buteo magni- 
rostris and the Faisdno Real of Mexico, for example). The bend of 
the wing may be lifted to cover part of the face. 

Influence of Instinctive Needs. Animals have become adapted 
to their environment and have developed a set of instincts fitting 
them to their efficient use of the surroundings (Chapters 10 and 23). 
While this may not be the most efficient use theoretically possible, it 
seems to be as efficient as needed in the battle of life. 

A bird that lives in the woods, like a Chickadee or a Nuthatch, does 
so because woods alone supply its instinctive needs for habitat. These 
birds have developed characteristic feeding habits, nesting habits, and 
behavior patterns adjusted to their way of life in the woods (Fig. 
11-4). The duckling, even a newly hatched one, seeks the water 
because water satisfies one or more of its instinctive needs. In the 
same way, other birds seek their characteristic habitats because only 
these satisfy their instinctive needs. Those not wholly satisfying in- 
stinctive needs, either correctly or by acceptable substitutes, will be 
used only proportionately for the fulfillment of these needs. They 
are marginal habitats which but partially supply the needs of life. 

Habit and tradition may play a part in the use of habitat, though 
their influence may be transitory only. The bird hatched and raised 
under one set of conditions may, when it shifts for itself, seek similar 
ones because of imprinting, though how much this controls the action 

f 1 1 1 1 JW *-J .y" ** -v-^ l ^.l M -.. lp , r .y*^^ 

of wi]g^bjrg is problematical. A possible source of testing this factor 
lies in comparing the behavior of socially parasitic birds. Young 
Cowbirds hatched in nests of Warblers, for example, could reflect 
their early life and perhaps act differently in a measurable way from 
others raised in Fringillid nests. But there seems to be no clear evi- 
dence to indicate that this happens, though it might be a fruitful study. 



Community Relationships. A community in the ecological sense 
consists of the organisms living together. It is hardly a "super organ- 
ism" as enthusiastic ecologists have said but includes only such plant 
and animal life as live in the same delimited area. Birds depend upon 
vegetation and lower forms, and an interdependence of organisms 
thereupon occurs in varying degrees. Thus, the insect-eating birds 
depend upon the insects which depend upon the plants. But plants 
depend upon only relatively few of the insects, sometimes upon none, 
and few insects actually depend upon birds for life success. Yet the 
presence of part of the community of organisms necessitates presence 
of those that mutually hold each other in check. In the sense of "all 
or none," it can be said that interdependence may often be rather 

As the breeding potential is so great, bird life necessitates destruc- 
tion of the excess; it may be said truly that animals depend upon both 
their enemies and their competitors for life success. Often "de- 
pendence" is but a small thing, as when the warning note of a Blue 
Jay calls attention to a marauding cat. Yet along the Florida coast, 
ecologists report that the presence of organic fertilizers from bird 
colonies is essential for the rich aquatic plant and animal growth in 
adjacent waters that in turn support the colonies (Alills, 1944). 

Carrying Capacity. It may seem facetious to remark that a bucket 
can carry no more water than it can hold were it not that this rather 
suggests the principle of carrying capacity as applied to birds, livery 
habitat has a limit as to how many birds may live there in safety. 
It may vary from zero in unsuitable habitat to the saturation point of 
the species in favored habitat. The saturation point is actually a func- 
tion of the bird; it is the maximum density to which they will go by 
themselves. In a sense, it measures the willingness of birds to be 
squeezed into small areas or into high densities. Carrying capacity, 
on the other hand, is a function of the land. But it is measurable only 
by the birds that use it; it cannot be measured above the saturation 
point, for that is the limit set by the birds themselves. 

Birds in the wild, being free agents, move about and occupy habitat 
of their own volition (discounting the fact that they may be under 
compulsion of instincts, such as that for reproduction). Birds already 
in a habitat may ignore newcomers or engage in conflict. Hence, 
carrying capacity is not the number that can be forced into an area 
but the number adjusted to the habitat and the toleration of others. 

For ordinary purposes, the carrying capacity may be considered 
as the maximum number (maximum biomass probably would be 
better, page 256) that an area can support adequately during the most 
severe or poorest time of occupancy the pinch period. For resident 


birds of middle and high latitudes, this usually is the winter. For low 
latitudes, however, it is usually the dry season. For desert birds, it 
would be the dry season or the winter. Carrying capacity thus is a 
function of the land. Good habitat with a favorable climate has high 
carrying capacity (according to species); poor habitat and habitat 
with seasonal extremes has low carrying capacity. The governing 
point is how well the habitat meets the needs of the species. The 
mobility of the birds, however, makes it possible for them in some 
instances to move in and occupy in great numbers during a short and 
favorable interval an area not otherwise wholly suitable. Some of the 
concentrations in the Arctic nesting grounds of many birds exemplify 
this. In a sense, it is shown also by all migratory birds. Concepts of 
carrying capacity would be adjusted accordingly. 


Seasonal Change. Change of season forms the dominant char- 
acteristic of the middle latitudes where large human populations live. 
Therefore, this great feature of the environment very properly may 
be given the most earnest consideration. But in so doing, we must not 
fall into the error of thinking that seasons in the low latitudes change 
in the same way as they do farther north (or south) . The temperature 
in equatorial regions may vary more during the day than does its 
average for the months of the year. The rainfall may be evenly dis- 
tributed or it may vary greatly from a marked dry period to a 
pronounced wet one. 

The variation in seasons rests on the fact that in its yearly pathway 
around the sun, the axis of the earth itself is tipped 23/2 from a per- 
pendicular with the plane of its orbit. Because the axis remains parallel 
to its own position at all times of the year, one pole will point toward 
the sun part of the time and away part of the time; meanwhile, condi- 
tions at the other pole will be just the opposite. The daily lengthening 
of sunshine and the angle of the sun will vary at all places throughout 
the year. Beyond the Arctic and Antarctic Circles (23^ in latitude 
from the respective poles) it will vary throughout the year from 24 
hours in summer to none in winter. The vertical rays will move from 
235/2 on one side of the Equator to 2 3 1 / 2 on the other (Tropics of 
Cancer and Capricorn). This shift of^23V-, in the sun's apparent 
position shifts the climatic belts evenlrTtne equatorial lands, where 
rainy and dry seasons may result. The cause thus is the same for 
winter-summer seasons in the high latitudes and for wet-dry seasons 
in low ones. Upon these seasonal changes rests a whole series of 
repercussions in the bird world. 


Phenology. Just as succession progresses in an orderly manner 
over the years, the change of seasons progresses systematically. To 
the progress of seasonal phenomena is applied the term .Jthenology . 
Phenological indicators are best known for the spring, ana trie ; oest 
known of these are flowering of plants, bird migration, and break-up 
of ice in lakes and rivers (Wing, 1943c). Less well known are insect 
emergence, fish spawning, and other seasonal events. 

Phenological indicators show a spring progression at an accelerated 
rate as the season advances northward. In like manner, an acceler- 
ation occurs along western sides of continents as compared to the 
eastern side (Hopkins, 1938). The relative earliness of spring may 
be shown by reference to phenology. Thus, a bird that nested April 
10 one year and April 1 1 the next nested earlier by one day as meas- 
ured by the calendar. But if the season were 5 days later on the 
second year, the April 11 nest day becomes phenologically 4 days 
earlier. The summer, fall, and winter seasonal progress can be meas- 
ured in a somewhat similar way, though techniques for marking the 
progress of winter are rather scarce. 

Influence of Light. Birds, with but few exceptions (e.g., Strigi- 
formes, Caprimulgiformes), are wholly diurnal. Their eyes accord- 
ingly have a nearly pure cone retina which provides keen daytime 
vision. In addition, red droplets (page 78) aid vision during the 
sunrise and sunset hours when light rays are red from the Rayleigh 
effect. The orange droplets act during morning and afternoon, and 
the yellow ones at midday. 

Most birds begin activity at or near sunrise and cease at or near 
sunset, though regular daily body rhythms related to photoperiodicity 
continue (Bissonnette, 1937). The time birds cease singing at night 
or begin in the morning bears a fairly direct relationship to light inten- 
sity; morning song will be later on cloudy or cool than on warm or 
clear days (page 333). As the season advances, morning song becomes 
progressively earlier, evening song progressively later (Fig. 11-5). 
The Wren (Troglodytes musculosus) began its song at light inten- 
sities of 0.5 to 1.4 foot-candles. The American Robin in Tennessee 
ended its evening song when the light ranged from 0.1 to 10 foot- 
candles, the Mockingbird between 0.1 and 176.8. Bronzed Crackles 
in Ohio are reported as leaving the roost when morning light reached 
about 14 foot-candles and Starlings at about 7.5. Lone Crackles not 
part of the social group left their roosts at lower light intensities than 
their fellows in flocks. A study of Herons at a New Jersey roost 
determined that Little Blue Herons, American Egrets, and Snowy 
Egrets arrived between 16 and 9 minutes before sunset, and Green 
Herons between 7 and 14 minutes after sunset. The Black-crowned 



Night Heron departed between 16 and 19 minutes after sunset. In 
the morning the same sequence occurred in reverse (Seibert, 1951). 

On cloudy days when the light intensity is low, birds may carry 
on activity all day long in a manner similar to the activity of morning 
and evening. Studies during solar eclipse show that the light intensity 
drop brings on evening song and even roosting, though the hour 
may be midday. 



<* 6.00 

P 5.20 
| 4.40 


























4.00 I 


i i 







) 20 30 


10 20 




twil ght 



:. .S 





* * 








>^ * 




102030 10 19 I II 21 31 102030102030 9 19 29 9 1929 8 18 

Fig. 11-5. The time at Washington, D. C., of first morning song of the 
Song Sparrow and Chipping Sparrow. (After H. A. Allard, "The First 
Morning Song of Some Birds of Washington, D. C.: Its Relation to 
Light," American Naturalist, 64(1930):436-469.) 

Light influences the habitat activities of many birds. Song Spar- 
rows avoid dense foliage and heavy canopies, though they forage in 
dark crevices, under stream banks, and under piles of vegetation that 
cast small shadows. Dark patches of willow have been reported to 
act as barriers. On the coastal marshes of California, vegetation 2 to 
3 feet high will be occupied by Song Sparrows; if only half that 
high, it will be used by Savannah Sparrows. The latter prefer higher 
light intensities (Marshall, 1948). 

The influence of day length in initiating breeding, migration, and 
seasonal events is covered elsewhere. 




Altitudinal Influences. Marked zonation of vegetation occurs 
with altitude and with it a reverberation in bird life (Chapter 10 and 
Figs. 10-7, 10-8). In general, the range of northern birds extends 




\ b 




\ e 











k x * 


















10 20 30 40 50 60 70 80 90 100 110 



S- 2 





, \ 


















\ + 









-40-30-20-10 10 20 30 40 50 60 70 

















^ h > 

^ / 

/ ^ 






^ / 






y S 






fOiDODrvJinoO ^J"n-OfO (O 



Fig. 11*6. Climatic characteristics of central Wyoming, (a) Lowest 
temperature of record, (b) mean minimum temperature, (c) mean tem- 
perature, (d) mean maximum temperature, (e) highest record tempera- 
ture, (f) days of "growing season" (g) start and end of growing season 
(mean above 42 F.), (h) days frost-free, (i) dates of last spring and first 
fall frost. (After Frederick 5. Baker, Mountain Climates of the Western 
United States, Ecological Monographs, 14(1944):223-254.) 

southward in higher altitudes and that of southern species extends 
northward along valleys. In the same latitude and within a few miles 
of each other, northern and southern forms will be found, though 
without ecological overlap. 


The most marked effects of high altitude upon birds are those of 
temperature, precipitation, and wind. The season may be short even 
in low and middle latitudes, for mountain climates vary with altitude 
(Fig. 11-6). High light intensity may be important also in clear 
mountain and plateau regions. The Mount Everest Expedition of 
1924 noted that small birds spent much of their time in the protection 
of boulders and depressions to avoid the fierce winds (Kingston, 
1926). Many of the birds concentrated around the native villages 
where protection from wind was available. A few lived in the en- 
trances to burrows of mouse hares. The Redstart nested at 17,000 
feet and the Lammergeyer soared at 20,000 feet. 

Probably important physiological adjustments to pressure occur in 
birds, but of this we are relatively ignorant. It is reported that the 
heart, especially the right ventricle, of mountain Ptarmigan is larger 
than that of lowland ones. Birds of high altitudes may have greater 
wing surfaces in proportion to body size than those of lowlands. 
Trees become gnarled, weatherbeaten, and shrublike near timber line. 
Often a tree forms a mat that may be but a few inches or a few feet 
high though many feet across. Exposed trees that grow erect usually 
have the branches sheared off to windward and tail-like to leeward. 
Around and under these, small birds such as the White-crowned 
Sparrow seek shelter. Even the White-tailed Ptarmigan prefers 
boulder-strewn mountain slopes to open ones. 

Similarity Between High Altitude and High Latitude. The con- 
ditions of high altitude result largely from cold and often may be 
desert-like. For general purposes, the high latitude and high altitude 
effects are alike, though the altitudes have high light intensity and 
high solar radiation in contrast to polar regions. The plants of moun- 
tain slopes in general are related to those of high latitude. The rarefied 
and clear atmosphere of high mountains (excepting cloudiness) admits 
more radiation than the atmosphere of lower slopes. This is especially 
noticeable with ultraviolet rays. The glare of sunshine upon snow in 
winter may bother birds as well as man, though by nature they may 
be better able to withstand it. 

Moisture and Humidity Influences. Because moisture determines 
how Truit^ .?P er g7 abundance 

and distribution of birds vary with moisture and humidity. Birds of 
the dry, desert regions tend to nest on the east and northeast sides of 
plants, which provide sun for the morning chill and shade in the 
afternoon heat. (As birds usually construct nests in the morning, this 
may only reflect construction-hour choice.) Within the desert, birds 
seek the thicker and more shade-producing shrubs during the heat of 


the day, so that a rhythm of movement may occur. Such protection 
reduces moisture loss. 

Though birds must contend with desiccation and heat in dry lands, 
they profit by the greater freedom from parasites, viruses, and other 
organisms of sickness. Disease becomes more important toward the 
wetter and warmer ranges, climate toward the drier and colder ones 
(page 400). 

Drought. Drought influence on birds occurs most frequently on 
the humid side of the dry lands in the middle latitudes. Another zone 
of importance may occur near the Tropics of Cancer and of Capri- 
corn. In historic times, drought conditions of the continental interior 
in North America have resulted in drying of marshes and pools, par- 
ticularly on the prairies. Drought in the season of young Waterfowl 
may leave them stranded, though adults can move out. The hatching 
success of many eggs is usually lower during periods of drought than 
in normal times. 

The production of seeds, fruits, and insects falls off during drought; 
green vegetation declines also. Many plants produce little food during 
drought periods and may even skip a crop year. Vegetation upon 
which birds depend during the nesting season may fail to reach the 
proper state and nesting may be poor. In the summer of 1951, for 
example, after nearly 2 years of drought in northeastern Mexico, the 
thorn shrub vegetation failed to leaf out and flower in a normal 
manner. In consequence, the great nesting colonies of White-winged 
Doves failed to materialize, and the birds failed to breed. Only a 
small portion of the birds came to the nesting sites in April, few birds 
came into breeding condition, and fewer built nests and laid eggs. 

During drought, emigrations of desert birds have been noted, such 
as of the Sand Grouse of Asia. But desert birds more usually react 
by remaining within their customary range and declining in vigor, 
reproduction, and activity (page 92). 

Reaction to Wind. The reaction of birds to wind has so important 
a role that it has been covered in previous discussions of other influ- 
ences. Little need be said except by way of summary and addition. 
The individual bird flies lower during wind, perhaps to take advan- 
tage of the slowing of wind by surface friction. Birds of weaker 
flight remain in bushes and trees, but powerful fliers may venture 
forth in heavy winds. Land birds rise into the wind and turn into it 
to alight. Water birds do likewise, though they may patter along the 
surface before becoming air-borne. 

Some sea birds nest on exposed sites because of the aid provided by 
wind in getting into the air. It is said that a level surface 


wind is preferred by the guano birds of Peru for nesting. Some sea 
birds live in windy zones because they perhaps depend upon wind cur- 
rents for travel. Albatrosses and others avoid zones of calm and fol- 
low the oceanic wind belts. Some sea birds possibly may circumnavi- 
gate the globe in the Southern Hemisphere by following planetary 

Soaring birds, such as Hawks and Vultures, depend upon winds as 
well as up-drafts of air. Along the sea coast, birds may travel oif the 
on-shore winds or along the up-draft from waves. The winds rising 
over ridges have been used by migrating Hawks. Hawk Mountain in 
Pennsylvania derives its fame from this fact. A number of peninsulas 
concentrate birds in a similar manner (page 308). Migrating birds 
drift with the wind, as explained in the chapter on migration. A num- 
ber of sea birds have been carried far inland by riding the winds of 
tropical storms. Some land birds likewise appear far off their path- 
ways because of wind and storm. 

The day-to-day life of the bird may be subject to wind action. 
Flimsily constructed nests, like those of the Mourning Dove, may be 
destroyed in numbers by high winds; at times this may be an important 
if not the most important limitation to nesting success. Birds feed on 
the lee side of woods and brush during high wind; often they become 
inactive altogether until the wind dies down. Ground nesters build in 
vegetation for wind protection, but the Short-toed Lark of Tibet 
builds a rampart of pebbles on the exposed side of its nest for wind 
protection (page 216). 



SCHMIDT, Principles of Animal Ecology. Philadelphia: W. B. Saunders Co., 1949. 
CLEMENTS, FREDERIC E., and VICTOR E. SHELFORD, Bio-ecology. New York: John 

Wiley & Sons, Inc., 1939. 

DICE, LEE R., Natural Communities. Ann Arbor: University of Michigan Press, 1952. 
*ELTON, CHARLES, Animal Ecology. New York: The Macmillan Co., 1936. 
HESSE, RICHARD, W. C. ALLEE, and KARL P. SCHMIDT, Ecological Animal Geography. 

New York: John Wiley & Sons, Inc., 1951. 
KENDEIGH, S. CHARLES, The Role of Environment in the Life of Birds. Ecological 

Monographs 4 (1934): 299-417. 

"LEOPOLD, ALDO, Game Management. New York: Charles Scribner's Sons, 1933. 
COSTING, HENRY J., The Study of Plant Communities: An Introduction to Plant 

Ecology. San Francisco: W. H. Freeman Co., 1948. 

ODUM, EUGENE P., Fundamentals of Ecology. Philadelphia: W. B. Saunders Co., 1953. 
WEAVER, J. E., and F. E. CLEMENTS, Plant Ecology. New York: McGraw-Hill Book 

Co., Inc., 1938. 
*WING, LEONARD W., Practice of Wildlife Conservation. New York: John Wiley & 

Sons, Inc., 1951. 


Territorial Relations 

of Birds 

In the folklore of the ancients, the spatial needs of birds and man 
alike very evidently assumed significance. Even though they had not 
the benefit of today's fund of knowledge (which tomorrow may seem 
scanty), observational ability among earlier peoples was not lacking. 
They demonstrated this in the sage saying that, "No bush will hold 
two birds and no roof can cover two women." We can hardly do bet- 
ter today, for it recognizes that wild birds and civilized man are two 
of the most territorially conscious of organisms. Like many other 
concepts of biology, that of territory may be interpreted often in writ- 
ings of an earlier day, just as in its folklore. But it would be folly for 
us to overlook the possibility that we today read ideas that were not 
theirs into the words of earlier people. 

Historical Development of Territorial Concepts. In writing of 
natural history in 1622, Olina (as reported in 1678 by John Ray) 
wrote: "It is proper to this Bird at his first coming to occupy or seize 
upon one place as its Freehold, into which it will not admit any other 
Nightingale but its mate." Olina, be it noted, added to this the words, 
"in which it ordinarily sings," though John Ray omitted them (Nice, 
1941b). The Swan-herders of Olina's time also recognized territory, 
for the Swan laws and regulations took note of the territory of the 
male in the quaint words forbidding interference with a Swan that 
"hath a walke." Gilbert White mentions bird territory in his famous 
Natural History ofSelborne (1789). In 1774, Oliver Goldsmith even 
used the word territory in discussing birds. 

Many another writer on natural history mentioned territory, usu- 
ally in speaking of a single species, clearly in recognition of the spatial 



needs of birds. Bernard Altum in 1868 discussed bird territory and 
wrote more completely of it than his predecessors (Mayr, 1935). He 
elaborated many principles of territory, most of them acceptable 
today. In this, at least, he went beyond his fellows who recognized 
the territorial demands of the birds they observed but who did not 
discuss territory as a principle. The organization of territory as a con- 
trolling principle of bird life must be credited to an English ornitholo- 
gist, H. Eliot Howard, who marshaled the basic concepts upon which 
others have built. 

The Definition of Territory. Though territory seems clear enough 
as a literary word, its use as a technical word in bird study has resulted 
in inevitable confusion of meaning. Some observers have defined ter- 
ritory as "an area occupied by one male of a species which it defends 
against intrusions of other males of the same species and in which it 
makes itself conspicuous" (Mayr, 1935). Certain other terms seem 
necessary to clarify the concept of space occupied by birds. The area 
over which an individual moves regularly will be considered here as 
its activity or borne range, which again is irrespective of age, sex, 
season, or place. It is likely that use of activity range for the area over 
which a bird wanders may not be understood by all, but this use of 
the term range is common in parts of the American West. It has the 
distinct advantage of applying as a general term to the space used by 
a bird. Its activity range is all the area that it uses regularly, its terri- 
tory the defended part of this range. At times the territory and range 
will be the same, but the territory of a colonial sea bird may be only 
a square yard at its nest, though its range may cover 100 square miles 
and more. The territory of a Mourning Dove may be only 2 square 
feet around the nest, though its range may extend 30 miles to a water 
hole. ^hz^nzittml^grpunti and feeding ground are art of a bird's 
range, though not p^JXfif its territory ". In an analogous way, the daily 
travel of a man to and from work covers much of his activity range, 
though the territory of highly territorial man may be only his house 
and lot and a few square feet of desk space at the office. The Com- 
mon Law recognizes that "a man's house is his castle/' which merely 
gives social and legal recognition to the principle of territory as a 
biological attribute of man. 

Kinds of Territory and Range. The breeding habits of birds often 
govern largely the types of territory taken up. But exceptions occur, 
particularly in species that maintain a semblance of territory the ^ear 
around. Some Mockingbirds regularly maintain winter territory, 
which often may be the same as that of the breeding season. Hence, 
breeding habits may not always determine the taking up of territory. 


"Winter territory" may be a flock territory, as happens sometimes in 
resident species that assemble into winter flocks, but flock range may 
be a better term. Territory may occur perhaps among birds in migra- 
tion, though this is still not clear. 

A basic classification of territory follows:* 

Breeding Territory 

Prenuptial display 

Pairing and pair maintenance 

Copulation (as among Sage Grouse) 


Feeding Territory 
Seasonal Territory 




A species may have territories that seasonally, sometimes simul- 
taneously, belong in more than one category. Communal territory (as 
reported in the Ani and Jackdaw) appears similar to the territory of 
a single bird or pair except that the group defends the territory. 
"Winter territory" of a Quail covey may likewise be "defended" by a 

Some types of territory do not as yet fit into any of the several 
classifications suggested. Perhaps all gradations occur between types. 

/ XN \ Guard 
Y BLUE\ and 
(w) (Pa ' r) ! 'eader 

f f - 
L 1 , 


. ' \ 



BLUE , \7-^; 
n / ^ 

40 ^ s~?\ { 


SNOW (R) /\f 

/ ~ 





r*- CANADA *| 

/ " 


1 J 


Fig. 12*1. Territories of Geese when moving (the letters indicate in- 
dividuals). The boundaries of such "moving territories" persisted 'when 
the group moved. Birds low in peck rights stayed in the rear. (After 
Dale W. Jenkins, "Territory as a Result of Despotism and Social Organi- 
zation in Geese," Auk, 61(1944):30-47.) 

* Armstrong (1947) discusses the various territory classifications proposed. 



Thus the living space of the Cowbird seems to be rather indefinite, 
though a pair may become dominant over others (Laskey, 1950). 
Our understanding of the space program of the Cowbird, however, 
is rather incomplete (page 235). Of unusual interest seems to be the 
space claims of some captive flocks; birds of a mixed flock of Geese 
have been reported to move with each bird occupying a particular 
moving position (Fig. 12-1). 

Territory In Colonial Birds. Birds nest in colonies for a variety of 
reasons, though no clear ones can be ascribed to some colonies. Sea 
birds particularly nest in colonies, which habit seems to have orig- 
inated in the physical scarcity of suitable rocks, headlands, islands, or 
cliffs. A reduction in territorial aspirations by the individual thus 
seems to be an advantage to the race living where a single island may 
serve for nesting by the birds inhabiting large ocean surfaces (Fig. 

Land birds whose habits make suitable nesting sites rather limited 
in number are those that occupy rocks, cliffs, or banks (e.g., Bank 
Swallows and Rock Doves), marshes (e.g., Red-winged Blackbirds), 
and buildings, a substitute for a rock cliff (e.g., House Sparrow, Star- 
ling, and Barn Swallow) . Perhaps the colonial nesting by Great Blue 
Herons and Boat-tailed Crackles reflects an ancestral trait of marsh 
birds. But colonial nesting by some birds, such as tropical Weaver- 
finches or the Passenger Pigeon of eastern North America, is not so 
readily explained. A very definite need for complete examination of 
the subject clearly is in order. 

The advantage of a reduction in territorial desire by many birds 
to the small surroundings of the nest manifestly has an advantage to 
the race (Fig. 12-3). From the standpoint of the race the Kingfisher 
with its small amount of feeding room along a creek may safely keep 
a bank to itself. But Bank Swallows roaming the countryside would 
not be able to use all of the great feeding space open to them if one 
pair monopolized a cut-bank. It seems obvious that one pair does not 
need a whole bank to itself. 

Seasonal Territory. The principal seasonal "territory" outside the 
breeding season is the winter individual range. The Ruby-crowned 
Kinglet may occupy a dooryard for a few days during migration and 
drive others away. The American Sparrow Falcon and Loggerhead 
Shrikes even in migrations show a spacing of individuals. But such 
spacings and intolerance, though rather common, perhaps should 
hardly be called territorial. 

The more solitary birds show a tendency sometimes to hold a 
winter "territory," though this may be merely a place where they 



12 feet 

/ i\i \ i 

! T _ 

, X X -^' ^W\ 

V5/ .*#/ \ 

8 I 

Fig. 12*3. Map of Herring Gull Colonies, Hat Island, Green Bay, 
Wisconsin. B, bird blind; S, shrub; R, rock; N, old nest; W, driftwood. 
(After Murl Deusing, "The Herring Gulls of Hat Island, Wisconsin" 
Wilson Bulletin, 51(1939):110-115.) 

stay by habit. A Blue Grouse may stay alone in one dwarf Douglas 
Fir but at times two may use the same one. White-throated Sparrows 
living in a brush patch may be rather intolerant during mid-winter. 
English Robins have been reported to be similarly intolerant toward 
intruders in their winter territory (Lack, 1946). The winter terri- 
tory of the Mockingbird has already been mentioned. Because of the 
indefinite nature of winter holdings, many writers prefer "winter 
feeding range" to winter territory. 

The roosting spot of a bird sometimes becomes relatively fixed; it 
may thus be considered as a special kind of seasonal territory. A 
Starling may return to the same spot on a branch; if several birds are 
removed, vacant spaces on the roost show their absence. Possibly 


American Crows also occupy the same sites at the roost; in any event, 
the entire roost forms a flock territory for the time being. 

Function of Territory. Territory probably serves several functions, 
its chief one in the breeding season being to aid breeding success. It 
may be argued with every evidence of truth that it has a survival 
value or it would have dropped out of the birds' make-up ages ago. 
Various ways in which territory operates have been put forward by 
field students. The psychological advantage gained by the bird on 
familiar grounds enables it to be dominant in its own territory. The 
pairings-off of birds may be more likely to succeed if one remains fixed 
and advertises as the male does, so that searching females have less 
trouble to find a mate. Probably the reduction in strife between birds, 
brought about by isolation on respective territories, aids breeding suc- 
cess. No doubt also, the protection of the individual, the nest, and the 
young by ownership of territory and exclusion of rivals or com- 
petitors is an important defense point. Nonmated birds presumably 
would interfere more with the sexual bond were it not for territory. 

Territory acts as a population limit in some species, though to what 
extent has not been established (page 228). The ultimate population 
will be governed in territorial species, however, by the number able to 
breed those successful in getting territories. In this respect, it pre- 
vents overpopulation and the resulting destructive forces. The aver- 
age territory of a breeding bird seems to contain more possibilities for 
food and nesting than are actually needed. The food supply is guar- 
anteed in normal seasons for a growing family when a territory is 
taken up. The carryover of territorial claims into winter perhaps may 
in part express habit, it has been suggested, just as humans find it easier 
to do repeated acts in the same way rather than to make new decisions. 

Selection of Territory. In general, the male bird selects the breed- 
ing territory. In the case of migratory birds, the male usually arrives 
somewhat earlier than the female, and males have territory in various 
stages of establishment by the time the females arrive. In the Phala- 
ropes, the reversal of sex habits carries over to the territory, which 
the female establishes. Among our common birds, the males that 
arrive first start to establish territories. Males arriving later often find 
the space divided up, so that they must strive to carve out territory 
from existing holdings, which usually are larger at the start than 
needed. Older males returning to their previous territories seem 
highly successful in ousting claim jumpers. Only males in breeding 
condition establish territories, though exceptions have been reported. 
Young birds await maturity before establishing territory, as occurs in 
those that take two or more years for development, but newly mature 


birds are less successful than others. In species that pair on the winter- 
ing grounds, or en route, members of a pair may participate in estab- 
lishing a territory. 

Usually the male defends the territory against intruding males only. 
Sometimes the female defends against intruding females, but a pair 
may defend the territory jointly. The females of Yellow-headed 
Blackbirds, a polygamous species, have been reported to take up a 
sort of subterritory within the territory of the male. In this species, 
the respective females maintain these subterritories against each other 
(Fautin, 1940). The Ring-necked Pheasant uses a similar subterri- 
tory system (page 349). 

Territorial Boundaries. The boundaries of territories shift con- 
siderably early in the season and may shrink very much before be- 
coming static. The female seems to have to learn the boundaries, and 
much effort by the male early in the season may be directed toward 
keeping her at home. 

Boundaries sometimes follow recognizable physical or vegetational 
features. One Vesper Sparrow may occupy the land on one side of a 
fence, another the other side; and the posts may be shared or con- 
tested. A creek may separate the territories of two Song Sparrows, a 
hedgerow those of Mockingbirds, and a row of trees those of Mead- 
owlarks. But often less clearly defined markers may serve. The 
boundary between the territories of two American Robins may pass 
across an open lawn, and one Meadowlark may use one side of a tree 
for singing, while another bird uses the other side. No doubt the birds 
learn to recognize the boundaries, even though they may be rather 
more unstable early in the season than they are later. 

Defense of Territory. Birds defend territory usually against others 
of the same species. Defense of territory, as has been said earlier, rests 
mostly with the male, though he may not be antagonistic toward 
intruding females (in which case the female may be). In general, the 
size of the territory may depend upon the ability to defend the 
boundaries. When birds of a species are less abundant, territory of 
an individual of that species is large. But increase in abundance tends 
to bring on more territorial defense than a bird can handle, and the 
territory shrinks accordingly. 

Territorial defense largely involves display and posture, usually by 
"intimidation," but failure of the intruder to retreat subjects it to 
physical attack (Figs. 12-4, 12-5). Early in the season the owner may 
be evicted by the intruder, but this seldom happens later. The owner 
pursues the intruder to the boundary and perhaps beyond. In case of 
neighbors, pursuit into the home territory of the intruder brings about 



a reversal and the pursued becomes the pursuer, which exemplifies the 
rule that "a male on its own territory is undefeatable" (Tinbergen, 
1936). Defense display involves puffing up the feathers, swaying, 
posturing, or turning color patches toward an opponent. Often 
threatening sounds accompany display; rushing back and forth may 
also be part of the performance. In general, the defense action is 
rather stereotyped for the species, though some individual modifica- 
tion may be in order (Fig. 12-4). 

Fig. 12-4. Display and dispute, .(a) English Blackbirds disputing terri- 
tory, (b) Raven threat display, and (c) Jackdaw submission display. 
(Adapted by permission iron? Bird Life, by Edward A. Armstrong, pp. 
29, 35, 51. Copyright, 1950, Oxford University Press, New York.) 

Fig. 12*5. Common Tern, (a) Threat posture by male to female, (b) 
threat posture of defending male at nest (left) to intruding male (right). 
Note the greater active threat position of the owner. (After R. S. Palmer, 
"A Behavior Study of the Common Tern," Proceedings of the Boston 
Society of Natural History, 42(1941):1-119.) 


Song and Territory. Because Chapter 17 will be devoted to song, 
it need be covered here only as it clarifies knowledge of territorial 
activities. Song serves a number of territorial purposes, the chief one 
being the advertisement of possession by the male. In so doing, it 
notifies females of the presence of an unmated male and signifies to 
other males the presence of an occupied territory. The male Bob- 
white ceases to sing when it acquires a mate, but a widowed male will 
sing again. 

Territories bereft of their owners usually will be occupied again 
in short order, probably in early morning (page 257), with subsequent 
territories tending to be in the same places as those of the predeces- 
sors. This substantiates belief that the habitat itself is all-important 
in explaining local bird distribution. The new males show a char- 
acteristic behavior pattern, distinct from that of established owners; 
the songs are louder and uttered more frequently, and the birds are 
more active in inspecting their newly acquired domains (Stewart and 
Aldrich, 1951). Song differences probably signal the state of affairs 
on the territory; human beings can recognize some meanings of songs 
and calls, though much remains unknown. 

Identification of Territory Limits. Boundaries usually may be 
identified by watching for boundary disputes and by following the 
daily travels of the birds, especially those of the males. The male 
moves about over his territory and by his songs an observer can locate 
his general territorial limits, but the decisive action always remains 
the boundary dispute. The various daily movements, actions, and 
disputes, when mapped, outline the freehold. In intensive studies, the 
birds may be trapped and marked for later identification as individuals. 
In most cases, repeated daily observations are needed before territorial 
limits can be identified with certainty. 

Size of Territory. Territory varies greatly in size among birds, and 
few general rules have yet been suggested. In noncolonial birds, it is 
said to increase with size of the bird itself. It tends to be larger among 
predators than among nonpredators, and larger for desert and grass- 
land birds than for brush and forest ones. But it is not possible to 
compute the size of territory by dividing the total area by the number 
of pairs, because territories may be set in an "unoccupied matrix" (see, 
for example, Fig. 11-2). Table 12-1 lists a few examples of sizes of 
territory reported. 

Territory and Population Limitation. Paradoxical though it may 
seem, territory appears to limit population to the number of possible 
territories on the one hand and to assure the habitat needs of a capacity 
population on the other. It has been said that territory is one of the 


Table 12-1 
Examples of Territory Size in Birds 

S P ecies (Acres, or stated) Reported by 

English Robin 2/5-2 Armstrong, 1950 

Ovcnbird l A-3 Armstrong, 1950 

Yellow Wagtail 1 l / 3 -5 Armstrong, 1950 

European Wren 6-7 Armstrong, 1950 

Song Thrush 3 % or less Armstrong, 1950 

Willow Warbler 1 A Armstrong, 1950 

Reed Warbler 200 sq. yds. Armstrong, 1950 

Golden Eagle Several square miles Armstrong, 1950 

Black-headed Gull 7 sq. ft. Armstrong, 1950 

St. Kilda Wren 3,000-5,000 sq. yds. Armstrong, 1950 

Zanzibar Red Bishop 900-1,000 sq. yds. Armstrong, 1950 

Song Sparrow % Armstrong, 1950 

Yellow-headed Blackbird 1,294 sq. ft. Fautin, 1940 

Mockingbird 1 Original data 

Wren-tit V 2 -l Yi Lack, 1946 

controls at the top of the food chain (page 443). The total breeding 
population will not exceed the number of territorial birds that can 
establish territories, though a floating reserve of unmated birds will 
exist and also have to be supported by the land. In the final analysis, 
territories (and hence the breeding of the species) are found only 
where the habitable holdings provide living needs. Because the size 
of the territory shrinks as the population increases, yet only to a lower 
limit (saturation point), it may measure roughly the capacity of the 
land to support a certain species (page 211). 


*ARMSTRONG, EDWARD A., Bird Display and Behavior. London: Lindsay Drummond, 

Ltd., 1947. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
HOCHBAUM, H. ALBERT, The Canvasback on a Prairie Marsh. Washington, D. G: 

American Wildlife Institute, 1944. 
HOWARD, H. ELIOT, Territory in Bird Life. New York: E. P. Dutton & Co., Inc., 


*LACK, DAVID, The Life of the Robin. London: H. & F. Witherby, Ltd., 1946. 
*NICE, MARGARET MORSE, The Watcher at the Nest. New York: The Macmillan Co., 


NICE, MARGARET MORSE, "The Role of Territory in Bird Life," American Midland- 
Naturalist, 26(1941) :441-487. 

NICE, MARGARET MORSE, "Studies in the Life History of the Song Sparrow," Trans- 
actions of the Linnean Society of New York, 6 (1943): 1-328. 


Social Relations and 
Social Behavior 

Although one unfamiliar with the wild might think nature always 
to be in a chaotic state, a characteristic commented upon by genera- 
tions of naturalists is its pattern. A scientific axiom holds that in 
nature, patterned behavior betokens the operation of order, not dis- 
order; of principles, not accidents; of laws, not chaos. The relation- 
ships of the several parts of the body show well the action of order 
in nature. The relations of a bird with its physical environment like- 
wise show order. Of equal interest, the relations of birds with each 
other those of the same or of other species show recognizable 
patterns. These all are considered loosely as forms of social behavior. 

Many activities of birds, such as those of courtship, might well be 
included in this chapter as forms of social behavior. But it seems 
useful to include here (somewhat arbitrarily) actions that concern 
other relations of birds with each other. In addition, a few miscel- 
laneous habits are included, largely for the reason that our knowledge 
of them does not fit them particularly well for inclusion elsewhere. 


Social Dominance. Groups of birds staying together by com- 
pulsion (as in caged birds), by flock habits (as in the Blue Tit or 
Bob- white), by communal courtship (as in the Sage Grouse), or 
habitat concentration (as in feeding areas), may develop a system of 
social rank. This social hierarchy rests upon the ability of one bird 
to dominate another. Dominance has been defined as the state of 
affairs in which one bird has "priority" over another; the first bird 



dominates the second. Birds like the Domestic Chicken, when held in 
close quarters, develop a peck order. The top-rank bird pecks 
chickens lower in the order than itself without return pecks; the 
second highest pecks all below it, and so on, until the lowest ranking 
bird defers to all and dominates none. The social order in a flock of 
White Leghorn hens is illustrated in Table 13*1. Confined birds in 
close quarters, like chickens, may develop pec k right, but those with 
more space may not. A rather similar type of dominance reported in 
winter flocks of the Black-capped Chickadee is practically unilateral 
(Hamerstrom, 1942). 

Table 13-1 
Social Order in a Flock of White Leghorn Hens 


Number Pecked 

Individuals Pecked 

BB .... 










RW . 







.... 4 








Source: W. C. Allee, "Group Organization Among Vertebrates," Science, 95 
(1942): 289-293. 

But a common form of dominance is peck dominance. In peck 
dominance, one bird dominates in a majority of the contacts, not com- 
pletely as in peck right. The association of position and territory 
seems to play a role in determining the outcome of these contacts. A 
bird in its own part of a cage or habitat seems dominant. In winter 
flocks of Black-capped Chickadees, for example, it has been noted 
that birds later establishing territory near the winter range of the 
flock stood high in the social order. This may have been because they 
were older birds or were actually on their home grounds, just as 
other birds on familiar grounds seem to have high prestige. The health 
of the bird may also influence its position, as may hormones (Allee 
etal, 1939). 

Age Dominance. In a group of birds, older ones may tend to 
dominate younger ones, although some immatures may at times domi- 
nate older birds. The first hatched in a nest where incubation begins 
at laying of the first egg have been reported to dominate later hatched 
ones, a condition that continued. It seems probable that some of the 
deferred maturity reported for several gallinaceous birds, Red-winged 


Blackbirds, and others, may represent age dominance by the older 
males over the first-year birds, originating perhaps in the low endo- 
crine level of younger birds. On the display grounds of the Sage 
Grouse, "master cocks" dominate "subcocks" and "guard cocks," 
though the age relation is not known for certain (Scott, 1942). The 
greater dominance of the older male has been noted in the establish- 
ment of territory (although it cannot be separated from the greater 
success of the bird in familiar surroundings) from which he usually 
ousts his younger rival, even though the latter became established 
ahead of the older bird. 

Sex Dominance. Males sometimes dominate the females prior to 
the breeding season, though the actual forcefulness varies greatly (Fig. 
13-1). After the sex bond has been well established, males may 
appear very solicitous of the welfare of their mates. There may even 
be reversals of dominance between sexes, as in the Snow Bunting 
(Tinbergen, 1936). In the everyday life of the pair, shifts of domi- 
nance between the male and the female may occur. Some females 
dominate at the nest. The male Mallard and the male Shell Parakeet 
are reported to dominate during the nesting period, though the female 
does at other seasons. In Geese, the female attains the social rank of 
her mate, and this may be the case also in several other species that 
flock together as mated birds. Preference of females for more domi- 
nant males has been reported in the Gambel Quail, Ruddy Shell Duck, 
and Domestic Chicken. But the more dominant Domestic Chicken 
may be trodden less than its subordinates, even though it may be 
courted more frequently. The entire matter of sex dominance is in 
confusion and in need of much study, especially in the wild. 

Species Association. Although evidence shows preference of 
one individual in a flock for others of the same species, most of these 
expressed preferences are toward birds of the opposite sex. Yet the 
general gregarious attraction for others of their kind forms the basis 
of the flock. 

Among birds of some habitats, such as of the forest, several species 
associate in seasonal (usually winter) flocks. This may be measured 
with an associational index (Dice, 1945a) by dividing the number of 
random samples in which one species occurs by the number of 
samples in which both occur together. In the Tropics, mixed-species 
flocks may associate together at various times of the year. Species 
associations show varying degrees of interspecies dominance. Nut- 
hatches dominate Chickadees and Titmouses.* In western America, 

* A common error among writers is to use titmice as a plural for titmouses. Tit- 
mouse comes from the words tit and mose, the latter from the Anglo-Saxon being the 
name for several kinds of birds. The name has no relationship to mouse or mice. 




Spring Dispersal Period 

period of 1 



r perlod of 1 

cock pairsj 


^period of 


display of <f 

toward o 

Fig. 13-1. Display and dominance of Ring-necked Pheasants (January 
through April). Broken lines represent infrequent occurrence, solid ones 
represent frequent occurrence. The shaded part of the figure indicates 
increase in size of testes. (After Nicholas E. Colllas and Richard D. Taber, 
"A Field Study of Some Grouping and Dominance Relations in Ring- 
necked Pheasants," Condor, 53(1951):26$-215.) 

the White-breasted Nuthatch dominates the Red-breasted Nuthatch, 
which in turn dominates the Pygmy Nuthatch. All dominate the 
Chickadees, but the Black-capped Chickadee dominates over the 
Mountain Chickadee (Wing, 1946b). The Ringed Penguin dominates 
both the Adelie and Gentoo Penguins. A male Ringed Plover has 
been reported to dominate the female Turnstone but to be dominated 
by the male Turnstone. And a triangular interspecies peck order has 
appeared among captive Cranes, Flamingos, and Pelicans, 



Building nests and caring for the young are so universally char- 
acteristic of birds that their absence is noteworthy. Loss of instincts 
for nest building, incubating, and caring for the young while foisting 
these duties upon foster parents marks a condition called social para- 
sitism. Five separate families of three different orders have evolved 
the habit, a case of parallel evolution of a separately originated habit. 
'About eighty-six species (not more than 1 per cent of the total of all 
birds) have the habit. The five families with numbers of species 
practicing the habit are Waterfowl (Anatidae) 1, Cuckoos (Cucu- 
lidae) 70, Honey-guides (Indicatoridae) 6, Icterids (Icteridae) 6, 
and Weaver-finches (Ploceidae) 3 (Friedmann, 1930). The Cuckoo 
and Honey-guide families show the most parasitism. More than half 
of their recognized species have the habit, which probably indicates 
the ancient nature of its acquisition. Not more than 1 to 7 per cent 
of the species in the other three families practice it. 

Cuckoo Parasitism. The habit of the Cuckoo to lay its eggs in 
the nests of another bird has been known since very ancient times. 
Aristotle (386-322 B.C.) commented upon it, and we may feel sure 
that it was common knowledge. Many writers since the days of 
Aristotle have written about the habit and offered a variety of theories 
accounting for it. One student of the subject remarks that the num- 
ber of theories "became almost as great as the number of writers on 
the subject" (Friedmann, 1930). 

The parasitic habits of the European Cuckoo are better known 
than those of any other Cuckoo or of any other socially parasitic 
species. The Cuckoo is a larger bird than its victims, but its egg is 
about the size of that laid by the host species. The European Cuckoo 
lays its eggs at intervals of 48 hours, although a few others do so at 
24-hour intervals (Baker, 1942). Each strain or race of Cuckoo has 
been reported to lay its eggs in the nest of a definite species, the eggs 
resembling closely those of the respective hosts. Cuckoo eggs vary 
in color from the reddish-brown eggs of strains laying in the nests 
of Meadow Pipits to bluish-green for those laid in Pied Flycatcher 
nests. They can usually be distinguished from the host's eggs, how- 
ever, by their more elliptical shape, slightly larger size, greater weight, 
and especially by the harder shell, gritty to the touch (Baker, 1942). 

The Cuckoo (usually the female?) establishes a* territory and 
watches for nest building by its host species. Nest construction by a 
victim seems to stimulate egg laying in the Cuckoo, much as nest 
construction seems to be necessary for continuation of the nesting 


cycle in the more usual breeding habit. The female removes an egg 
in her bill, previous to laying her own egg, but not until she visits 
the nest for laying her own. Once having laid, she pays no further 
attention to the nest. 

The European Cuckoo lays but one egg in the nest of a host species, 
and its incubation period rather closely matches that of the host 
species, so that the Cuckoo and the natural young hatch at about the 
same time. By combination of instinctive behavior and an anatomical 
depression or cavity in the back (called a "diabolical combination" by 
one writer), the young European Cuckoo ejects the rightful occu- 
pants over the edge of the nest to their doom. The young Cuckoo 
works itself under the other nestlings during the first few days, before 
its own eyes are open, and one by one elevates them to the rim of the 
nest and on over the top. All Cuckoos of species having young mark- 
edly larger than their nest mates seem to do this, which leaves the 
young Cuckoo to be raised alone by the foster parents. Cuckoo 
species laying several eggs in a nest and with young no larger than 
the true occupants seem not to have this habit. 

American Cowbird. In contrast to the Cuckoo, young Cowbirds 
do not eject their nest mates from the nest. By sheer bulk, however, 
the young Cowbird may crowd out the young of small birds. The 
eggs of Cowbirds are not specialized for each host species, nor do 
the Cowbirds remove eggs without piercing them. The Cowbird 
seizes the egg in its bill, the two mandibles making small holes (Chance 
and Hann, 1942). The Cowbird removes one egg from the nest to 
be parasitized during the afternoon of the day before laying, less 
often the day of laying, and rarely the day after. Eggs are removed 
only when two or more are present (Hann, 1941). 

The female watches nest building intently and lays eggs from 1 to 
5 days later. She spends from a few seconds to a few minutes at the 
nest and flies away immediately after laying. Of note is the fact that 
the Cowbird has developed the habit of laying at early dawn, usually 
within a half hour of the time that it begins to get light (Fig. 13*2). 

The Cowbird lays eggs in "sets" of about five each. This gives three 
"broods" for the breeding season, which corresponds to the normal 
successive nestings of other birds. Contrary to general belief, adop- 
tion of a parasitic life has not interfered with the mating bond, and 
Cowbirds practice monogamy (Laskey, 1950). 

Origin of Social Parasitism. No satisfactory theory has been 
devised for explaining the origin of social parasitism. Theories that it 
originated in polyandrous habits or unbalanced sex ratios (males of 
parasitic Cowbirds are said to outnumber females) are untenable for 



Fig. 13 2. Coivbird laying in a Red-eyed Vireo nest at 4:45 A.M., 
Butler, Pennsylvania, June 11, 1945. (Photograph by Hal H. Harrison.) 

the monogamous American Cowbird as well as for the monogamous 
Vidna macroura (a Weaver-finch of Africa). Usurping a nest has 
been suggested as an initial stage (page 242). It has been predicated 
also that the parasitic habit may have arisen from a lack of coordina- 
tion between the egg-laying and nest-building instincts, resulting in 
eggs being ready before the nests had been completed. The chance 
laying of eggs in the nests of other birds by a female stimulated by the 
sight of the eggs has been suggested also as a beginning of the habit 
(Allen, 1925), but this must be accompanied by or preceded by a 
marked reduction in attachment of the bird to its own nest (Fried- 
mann, 1929, 1930). 

The Honey-guides, like the Cuckoos, seem to be a family that 
never had highly developed building habits, which habits would there- 
fore be easier to lose, it is presumed, than highly developed ones. The 
nest building of the Icteridae, however, is highly developed, as is that 
of the Ploccidae, both of which have parasitic species. The highly 
developed nature of the nest building of these two families indicates 


that the change responsible for parasitism was an internal (physio- 
logical) and not an external (environmental) one (Friedmann, 1929). 


Gregariousness. A rather high proportion of birds shows gre- 
garious habits during the nonbreeding part of the year. It can be 
said with realism that this tendency for gregariousness characterizes 
the bird. 

In the middle and high latitudes, the solitary and "asocial" nature 
of the breeding season gives way rather early to the flocking of the 
migration season (page 312), though it may persist over much of the 
summer. The core of the flock in many small birds seems to be 
formed by the broods of young that gather in favored feeding areas 
or wander about to unite with other wanderers. Late nesters appear 
to be late flockers. Among some species, adult and young tend to 
flock together, though in others, many adults may be tending to late 
broods or nests. In a few others, the adults flock together and migrate 
separately, sometimes ahead of the young. 

Male Ducks of the marshes desert the females and band together 
in summer; these bands seem to constitute a nucleus for later flocks. 
The birds that have congregated in protected marsh areas for molting 
of the flight feathers form another nucleus, and the broods of young 
make up still a third. Among Geese, Swans, and Cranes the family 
stays together but joins with others for migration, so that a flock 
of such birds may become an aggregation of family units. 

The role of preference for familiar birds ("friends") and of site 
tenacity in holding flocks together is not known. It has been shown 
that some birds know each other as individuals (page 348). But 
colonial birds may form rather fixed nesting groups because of site 
tenacity, a behavior trait of the Common Tern modified by another, 
trait termed group adherence (Austin, 1949). The bond holding to- 
gether some of the social birds, like members of the Weaver-finch 
family, may be of this nature. 

With the advance of the breeding season, the gregariousness breaks 
down into the isolation tendency of the territorial birds (Chapter 12). 
Colonial birds show much less of this breakdown, being territorial 
largely in respect to the nest site itself. Some land birds in the nesting 
season show a definite gregarious tendency not of a colonial nature. 
But colonial birds clearly show their gregarious nature in the breeding 
season. The Passenger Pigeon nestings involved millions of birds. 
Many species of Dove nest in colonies (e.g., White-winged Dove 
and Red-billed Pigeon) but others may be solitary nesters. 


Migrating Flocks. The way in which the flock flies is a character- 
istic of the species and to some extent of groups. It is related also to 
habitats. American Robins drift across the sky in loose flocks, while 
the drift of New World Blackbirds and Crackles is somewhat more 
compact. But both may string out as far as can be seen. In a sense, 
they are aggregations of flocks (sometimes called flights) rather than 
an individual flock. Shorebirds tend to form into compact flocks tfyat 
may wheel about the sky in a "drill formation." As they twist in 
unison, they often flash light and dark as the light underparts or dark 
backs turn to the observer. Starlings, Waxwings, and a few other 
land birds may do so also. Brown Pelicans and others fly in a 
V-shaped flock, a formation that lets each bird utilize some of the 
energy lost by the lead bird in forming vortexes off the wings 
(Storer, 1948). It also gives vision ahead, though this seems not to 
be an important item. 

Size of Flocks. The size of bird flocks varies conspicuously, which 
probably roughly indicates gregariousness. Birds may associate to- 
gether because of the gregarious urge or congregate together for 
reasons of habitat (Wing, 1941). The former probably constitutes a 
true flock, the latter a congregation. The smallest size of a flock would 
be two birds associated together (as distinguished from a mated pair). 
Two White-breasted Nuthatches in winter show awareness and interest 
in each other by their calling back and forth, and the two would form 
the minimum flock. 

Table 13-2 
Size of Bird Flocks in the Christmas Censuses 


Number ' 

Number of Birds 
Average Flock 




Downy Woodpecker 

, 407 


American Crow 



Black-capped Chickadee 



\Vhite-breastcd Nuthatch 




, 487 


House Sparrow 

, . . . 394 





Slate-colored Junco 



Tree Sparrow 



Song Sparrow 

. . 326 


Migratory birds gather into larger flocks than resident ones; the 
flock of semimigratory birds falls between the two. Thus, 6,492 flocks 
of Passerine birds in winter averaged 18.6 birds for the migratory 


species, 10.3 for the semimigratory, and 8.0 for the resident ones.* 
Data for birds with more than 300 flocks each during the Christmas 
Census season in the United States and Canada are given in Table 13-2. 

Size of Flock and Habitat. Birds, like mammals, show greater 
gregariousness in the open range environment than in the brush or 
forest. In fact, few birds of the forest ever form large flocks except 
for those that range widely over the forest canopy or when away 
from the forest in winter, as shown, for example, by the Bohemian 
Waxwing. A comparison of 6,283 flocks of small birds at Christmas 
time indicates clearly the working of this rule of differential gre- 
gariousness (Table 13-3). 

Table 13-3 
Variation of Flock Size with Habitat 

Number of Size of 

Flocks Average Flock 

Forest birds 2,926 4.1 

Brush birds 1,841 8.6 

Birds of the open 1,516 16.9 

The Bob-white shows a habit, which may be of wider occurrence 
among birds, of going in larger flocks in the colder parts of its range. 
The covey averages about a dozen birds (average of 11.9 for 1,362 
flocks in the Christmas Censuses) . But flocks of the warmer regions 
average about two fewer than in the coldest part: 

North Central 12.5 South Central 1 1.0 

Northeast 11.3 Gulf States 10.8 

Among families of North American birds, the Icteridae, Anatidae, 
Laridae, and Corvidae lead in gregariousness as measured by flock 
size. At the other extreme, the least gregarious, as measured by flock 
size, are the Falconidae, Accipitriidae, and Strigidae. The most gre- 
garious species, as measured by winter flock size, seem to be the Cow- 
bird, Red-winged Blackbird, Canada Goose, Starling, American Crow, 
and Coot. The most solitary are the Northern Shrike, Barred Owl, 
Screech Owl, Goshawk, Sharp-shinned Hawk, and Cooper Hawk. 
The most solitary birds are predaceous ones; hunting for prey on land 
evidently does not lend itself well to flock effort. 

Advantages of Flocking. It would seem obvious that an im- 
portant manifestation of life like the flocking habit would have a most 
important advantage to the species and to the birds practicing it. 
Many advantages have been suggested, and doubtless they vary from 
species to species and from place to place. 

* Original data. 


The advantages of flocking to the small prey species as a measure 
of protection from an enemy seem to be many, though considera- 
tions of predator-prey relations tend to overemphasize the role of the 
predator in the life of the prey. A flock of feeding birds almost always 
has one or more birds looking up from feeding, who would thus be 
able to detect and to give warning of an approaching enemy. A 
predator making a dash into a flock free to escape can become con- 
fused by so many similar birds and fail to concentrate enough on one 
bird to catch efficiently. The response of a flock to the distress call 
of a bird often is real, and the attacker may be harassed or literally 
mobbed by his prey. The soaring Turkey Vulture sights another 
dropping to food, and by following it in turn notifies others so that soon 
a number from several miles around will be attracted to a dead animal. 
The Vultures, however, should be considered as essentially solitary in 
their soaring, though roosting in groups. A Carolina Chickadee find- 
ing a suet station for the first time calls in a voice that seems inter- 
pretable as notifying others of the food, for others come quickly to it. 
Foraging by a flock spread out over considerable area gives more 
probability of locating any food present than if the birds went alone. 

A lone bird or a small flock of some species shows a nervousness 
not found in a large flock, and it may be assumed from this that the 
normal size flock may satisfy better the gregarious nature. Birds seem 
to have some facility for judging numbers, for some species almost 
never travel in small flocks in winter. 

Of special interest are the few examples of flock continuity gener- 
ation after generation. A number of cases have been reported of 
flocks of birds (such as House Sparrows, in Chapter 20) having an 
albinistic trait show up for a number of years. Individuals hatched, 
lived, and died, yet the structure of the flock held together strongly 
enough to maintain the sprinkling of albinism. 


Mutualism. Several habits of birds have been given various names 
from time to time, such as cooperation, tmitualism, coitrwensalisw, 
symbiosis, and alliances. Thus, the American Cowbird attends upon 
cattle and horses (but formerly bison) for flies and other insect life. 
The Cattle Egret of Africa performs a similar duty for the African 
buffalo and elephant. Several birds of the Shorebird group have been 
noted to dart in and out of the open mouths of basking crocodiles, but 
whether to feed upon food remnants, leeches, flies, or gnats has not 
been established. Ancient writers thought that they fed on leeches as 
a sort of favor to the reptile. The Ox-pecker, sometimes called also 
Rhinoceros Bird, feeds around cattle, camels, antelopes, and rhinocer- 


oses of Africa somewhat as do the Cowbirds of America. Occasionally 
its habit of picking ticks may lead to picking at wounds (perhaps 
caused by ticks) to bring further injury to the animal. The Rosy 
Bee-eater of East Africa rides upon the back of the Bustard, evidently 
using the larger bird as a walking substitute for a bush. From this 
perch, it sallies forth to capture insects and return to its perch. 

Many cases of apparent association of small birds with larger or 
more pugnacious ones have been reported, though it is difficult to 
separate intentional association from the habitat preference which 
throws the birds together. In parts of the American West, nests of 
the Raven frequently arc placed near those of the Prairie Falcon. But 
a probable explanation may be that the Falcons often use old nests or 
nest sites of Ravens and other birds, and Falcons and Ravens alike 
tend to return to the same nesting area year after year. Because cliffs 
with suitable ledges are necessarily limited in number, some closeness 
of nesting may be expected. 

Small birds have been reported nesting amidst the sticks of larger 
birds' nests. Some reports seem to show that the smaller bird seeks the 
nest of the larger one, but others indicate that the smaller bird merely 
selects what proves to be suitable to it, not necessarily because it is a 
predator's nest. But some birds of the Tropics seem to pick nest sites 
at or near the nests of pugnacious birds like the Flycatchers. A few 
may place nests in acacia bushes which are also the homes of acacia 
ants living in the hollow thorns. But it seems highly improbable that 
the birds do so in order to take advantage of the stinging of the ants 
as a defense against possible enemies. One should always bear in 
mind, however, that bird studies in remote places suffer from their 
short-time nature. 

The habit of the Burrowing Owl to live in burrows in prairie dog 
towns has long been commented upon. But the harmony of rodents, 
birds, and rattlesnakes popularly envisioned does not occur. The 
Burrowing Owl may use a burrow for many years. One in Douglas 
County, Washington, was occupied by Burrowing Owls from 1902 
to at least 1939 (Jellison, 1940). 

Highjacking. An activity of some birds in pirating food from the 
rightful hunters has been called parasitism, but a far better term for 
such brigandage is the American word highjacking. The several 
species of Jaegar and Skua intimidate Gulls that have caught fish and 
force them to give up the catch. Gulls in turn sometimes highjack fish 
from Terns and Pelicans. The Frigate or Man-o'-war Bird likewise 
forces Gannets, Pelicans, Cormorants, and others that catch larger 
fish to give up the catch, but the Frigate seems to do this largely in 
calm weather, a time difficult for its type of flying and foraging. The 



Fig. 13 3. The act of anting as done by a Song Sparrow. (After Mar- 
garet Morse Nice and Joost Ter Pelkivyk, " 'Anting' by the Song Spar- 
row," Auk, 51(l940):52Q-522.) 

Bald Eagle feeds upon fish that it catches for itself, but it also high- 
jacks from the Osprey. 

Another form of highjacking has been noted in the stealing of 
nests of birds. Thus, the Horned Owl has been known to dispossess 
Bald Eagles, Red-tailed Hawks, and American Crows. But among 
some species, usurping the nest of another replaces normal nest build- 
ing. The Bay-winged Cowbird of Argentina and other parts of South 
America nests late, after most other birds have finished, when empty 
nests that it can use are most abundant. But pairs of Bay-winged 
Cowbirds frequently fight with other birds and take possession of 
their nests, ejecting any eggs or young present. In most cases, they 
do some repair to the nest. If no nests are available, either by piracy 
or by taking over abandoned ones, the Bay-winged Cowbird may 
build a nest, which shows that they still possess a latent nest-building 
instinct brought into play when other means fail (Friedmann, 1930). 
The Bay-winged Cowbird cares for its own young, but in turn is 
parasitized by a relative, the Screaming Cowbird. The latter is de- 
rived phylogenetically from the former, and the usurping of another 
bird's nest may be an initial stage in the evolution of social parasitism 
(Friedmann, 1930). 

Anting. Birds have been observed to frequent or bathe in ant hills 
and to dress their feathers with ants. The act may vary from placing 
the ants under the wing to actually rubbing the ant into the feathers. 


^Several substitute actions, some involving pungent berries or other ob- 
jects, have been reported. The purpose of this action has not been 
explained, although claimed to be some form of parasite control by 
the formic acid or other acids in the bodies of the ants (Figure 13-3). 

Wing Flashing. Mockingbirds feeding or hopping about a lawn 
often pause between runs and lift the wings in a performance called 
wing flashing (Sutton, 1946). Females usually practice the act, 
though rarely will a male do so; young birds hardly able to flutter 
about a lawn may do so also. No explanation has suitably accounted 
for this practice. It may be part of a food-gathering performance, an 
effort to make insects reveal themselves, or an instinctive gesture of 
wariness. The fact that females and young do most of the flashing 
has suggested also the explanation that it is some signal appropriate to 
identification of sex and immaturity. 

Similar acts have been reported for the Brown Thrasher, Road- 
runner, and Least Bittern. That of the Road-runner and Least Bittern 
seems related to food gathering, though that of the Mockingbird may 
not be. The related Catbird (possibly only the female and young) 
have a performance involving a rapid, shivering wriggle of the wings. 
It appears to be a part of or related to courtship and nuptial display. 
It has been suggested that the wing flashing of the Mockingbird is a 
slow-motion analog of this performance. 


ALLEE, W. C., Cooperation Among A'rimwls. New York: Henry Schuman, Inc., 1951. 

* ARMSTRONG, FDWARD A., Bird Display and Behavior, London: Lindsay Drummond, 

Ltd., 1947. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
BAKER, E. C. STUART, Cuckoo Problems. London: H. F. & G. Witherby Ltd., 1942. 
CHANCE, EDGAR P., The Truth About the Cuckoo. London: Country Life, Ltd., 1940. 

*DARLING, F. F., Bird Flocks and the Breeding Cycle. Cambridge, England: Cam- 
bridge University Press, 1938. 
*EMLEN, JOHN T., JR., "The Study of Behavior in Birds," in Recent Studies in Avian 

Biology. Urbana, 111.: University of Illinois Press, 1955. 

FRIEDMANN, HERBERT, The Coivbirds. Springfield, 111.: Charles C Thomas, 1929. 
HOWARD, H. ELIOT, An Introduction to the Study of Bird Behavior. Cambridge, 

England: Cambridge University Press, 1929. 
*LORENZ, KONRAD Z., King Solomon's Ring: New Light on Animal Ways. New York: 

Crowell Publishing Co., 1952. 
RICHDALE, L. E., Sexual Life of the Penguin. Lawrence, Kan.: University of Kansas 

Press, 1951. 
SCHELDERUP-EBBE, T., "Social Behavior of Birds," chap, xx in MURCHFSON, Handbook 

of Social Psychology. Worcester, Mass.: Clark University Press, 1935. 
TINBERGEN, N., "Social Rcleasers and the Experimental Method Required for Their 

Study," Wilson Bulletin, 60 (1948): 6-51. 
TINBERGEN, N., The Study of Instinct. London, England: Clarendon Press, 1951. 


B/rcl Abundance 

Like many other words in common use, abundance and population 
may mean different things to different people. Both have various 
shades of biological meaning, some definite but others indefinite. In 
general, the word population has the more exact meaning in the bio- 
logical field. Among bird students, it may mean ( 1 ) all the birds of a 
geographic area, (2) all the individuals of a species, (3) the individuals 
of one or more species in a habitat, (4) an interbreeding group of 
birds of one species, or (5) a group of individuals forming a subgroup 
within a species unit, often in a genetic sense. 

An important part of any abundance study is the relative numbers 
of males and females, and of adults and young constituting the whole 
(Chapter 8). Birds may be of any age from newly hatched young to 
oldsters. It may be assumed from the practical standpoint that no 
birds past the breeding age survive long or make up any important 
part of the population. Figs. 14-1 and 14-2 show sample analyses of 
populations in the wild. 


Environmental Population Influences. Any environmental fac- 
tor that influences survival, presence, or numbers of birds is reflected 
in the problems of bird abundance. The salinity of ocean water af- 
fects the composition and abundance of plant and animal life, which 
in turn may influence bird life. Because the oxygen of the water, the 
fertility of the soil, or the temperature of the air controls so markedly 
the success of the plant and animal life, we can hardly limit considera- 
tion of the environment as a population influence or even as a popu- 
lation control. This discussion, however, considers other manifesta- 
tions and leaves the environmental influences largely to other sections 
or to the suggested readings. 



Fig. 14*1. Analysis of the population curve of the House Wren in 
respect to age and sex and correlated with the wean December-February, 
'whiter-range temperature. (After S. Charles Kendeigh, Measurement of 
Bird Populations, Ecological Monographs, 14(1 944) :6l -106.) 


en 160 
g 140 
m 120 

u 80 

1 60 
i 40 


Immature: Adult ratio in November 


1935 1936 1937 1938 

Fig. 14*2. Age composition and cha?iges in numbers of a Valley Quail 
population at Davis, California, as determined from banding. (After 
John T. Emlen, Jr., "Sex and Age Ratios in Survival of the California 
Quail," Journal of Wildlife Management, 4(194Q):92-99.) 



Population Measurement. There are several ways of determining 
and indicating bird populations; for most purposes these may be di- 
vided into four convenient groups: (1) total enumeration, (2) enu- 
meration by classes, (3) sample counts, and (4) relative abundance 
counts. As would be expected, the four groups listed are not mutu- 
ally exclusive. Bird students commonly call counting methods cen- 
suses, though the term may have somewhat different meanings in 
other fields. 

Ornithologists seldom enumerate all the species of an area or all 
the individuals of a species except on sample areas, which enumera- 
tions correctly are considered as sample counts. Yet a few such total 
enumerations can be made on occasion. Bird students in Michigan have 
made counts of the small population of Kirtland Warblers and con- 
clude that the total number does not exceed one thousand (Mayfield, 
1953). A practically total count of the Greater Snow Goose is possible 
at its resting grounds on the St. Lawrence during fall migration. In 
the same way, the Great White Heron, Whooping Crane, and several 
other remnant species may be counted in their entirety. Birds that 
nest in colonies, such as Herons, Pelicans, Gulls, and many others can 
be counted if the range is not too large (Fig. 14-6). It is likewise 
possible to enumerate all the birds of an island or other similar area 
so as to have a total, not a sample count. Enumerations of this na- 
ture result in valuable bird population data. 

Enumeration of birds by classes commonly takes the form of sex 
counts or age counts. Often such counts are obtained as a by-product 
of some other activity, among which the most common are trapping 
for banding, checking the game birds killed by hunters, and counting 
birds in flocks. Successful counting by classes of live birds in the 
field requires that the birds be distinguishable at a distance. Sex 
counts of birds in flocks, as in Waterfowl, take two persons, an ob- 
server calling off to a recorder ( Yocom, 195 1 ). It is possible, however, 
for one person to count the birds of one sex and then to count the total 
flock in a second operation. 

The commonest sample counts take the form of censuses over 
predetermined areas. These may be of a single-cover type or of an 
area embracing several types. Although the taking of counts would 
seem to be a relatively simple procedure, biometricians indicate that, 
except for counts based upon territory determination, a rather large 
number of variables weakens the statistical validity of many of the 

Relative abundance counts gather data not as total counts but in 
relation to some unit of time or space (Chapter 24). The commonest 
ones are birds observed relative to hours, days, weeks, months, field 


trips, or distance. The data, usually appear as Jrirds fcer h&w.Qr birds 
per wile or as a frequwicy. of. occurrence. Relative data may also be 
obtained by sounds, movement, or other manifestation indicating 
abundance. Thus, one may make a song count of Woodcocks or 
drumming count of Ruffed Grouse. 

All population determinations are subject to the influence of 
flocking and seasonal characteristics. As discussed elsewhere, gre- 
gariousness reaches its lowest ebb in the breeding season, though even 
then groups of unmated birds or even late spring migrants will be 
found. Even the family groups give trouble in field counts, especially 
in species like the Black-capped Chickadee whose young may not 
necessarily be distinguishable from adults. Adding to the complexity 
of the problem are differences as of habits, conspicuousness, cover 
choice, daily rhythm, seasonal rhythm, and identification case (Ken- 
dcigh, 1944). 

Breeding Potential and Environmental Resistance. Every bird 
has the capacity to reproduce itself, which, on theoretical grounds at 
least, seems always equal to the loss caused by environmental pres- 
sures. The idealized reproductive capacity is termed the breeding po- 
tential and the reduction features of the environment, environmental 

Though we think of the breeding potential as a fixed biological 
characteristic of the bird itself, some variation does occur. When 
populations are high, the number of eggs and even the number of 
broods may average fewer than when populations are low, in accord 
with the principle of inversity or inverse tendency. Birds in various 
parts of their ranges may lay larger or smaller numbers of eggs to 
a set, or they may nest more often or less often during a season (Chap- 
ter 18). While it may be argued that these elements are environ- 
mentally induced, it seems best to term them internal adjustment fac- 
tors. The relationship among the three gives the population, which 
may be expressed as the relationship: 

The breeding potential represents the factors of number of eggs 
per nest, number of nestings each season, and number of reproductive 
seasons in the life of the bird. A species that averages four eggs, nests 
three times a year, matures in 1 year, and whose life averages 4 years 
(through four nesting seasons) would have a breeding potential of 
forty-eight for each pair. But one of the same laying capacity and 
age but taking 2 years for maturity would have a breeding potential 
of thirty-six for the same years. Environmental resistance foredooms 


forty-six attempts of the one species to failure and thirty-four of the 
other. Fig. 14-2 illustrates actual records of the "saw-tooth" nature of 
the yearly fluctuations in population. Fig. 14-3 shows the concept of 
an idealized yearly curve or population in a "stable system." If the 
environmental resistance becomes greater temporarily, population de- 
clines; if permanent, the population establishes a new, lower level. 
Continued increase of environmental resistance (or too great an % in- 
crease) may mean decline to extinction. A reduction in environmental 
resistance means an increase in population, permanent or temporary 
as the case may be. But population increase is self-limiting because of 
space needs and other internal factors, so that continued increase does 
not occur for long. 

Adults and Year Year Year Year Year 

newly hatched 


Fig. 14*3. Idealized curve of population to show the rise to a peak 'with 
the arrival of young in the nesting season and decline to previous level at 
the corresponding period next year. This zig-zag curve is the normal thing 
in nature (compare with Fig. 14.2). 

Some pertinent generalizations may be made, though we must recog- 
nize the limitations inherent in our knowledge of breeding in the 
wild. The breeding potential in part actually reflects the breeding 
effort necessary for replacement of the parents (and birds dying 
without successors), a factor recognized under the general term of 
life hazqxds. The pair of birds with forty-eight tries would take four 
years to replace itself, whereas the one with thirty-six tries takes three 
years. The yearly effort of twelve each is the same and the net re- 
placement the same the number of parents. Probably a representa- 
tive measure of the reproductive potential is the net possibilities for 
a common period of effort, such as efforts needed for a century of re- 
production. Both birds in the example would lay 1,600 eggs, one 
would have twenty-five generations, the other thirty-three. But for 
fifty generations, the former bird would lay 3,200 eggs and the latter, 

Table 14- 1 indicates possible c^succej^r^tj^s.. It also gives a very 
real idea of the gaps in our ornithological knowledge. Except for the 
number of eggs and possible number of nestings each year, the data 
are largely conjectural. We do not know the number of years of 
reproductive life; some birds do not mature until the second or third 


year (perhaps even longer). But the general pattern seems logical, 
though our knowledge of it will surely improve. 

Table 14-1 
Possible Expected Egg Success of Birds 

No. of Nests Each Laying 
Eggs Year Seasons 


Egg Success 



[ 4 
L 6 
! 2 


















Mourning Dove 

Passenger Pigeon 




"Passerine birds" 

4 : 

In general, it may be said that species laying large numbers of eggs 
do so because the expected egg success is low. The Passenger Pigeon 
which laid only one egg, on the contrary, had a high chance of hatch- 
ing and maturing that egg (and others of its laying life) into a bird to 
succeed its parents. It is likely that a Crane laying for ten seasons 
(probably they lay for more) has a potential reproductive success for 
its eggs not much greater than that of a Passerine bird. But the fact 
that the young take several years for maturity introduces age-class 
complications. Age itself increases life hazards, which may be addi- 
tional factors in low egg-success expectancy. A low turnover per- 
haps does not supply the "biological vitality" of a rapid one. Such 
likely variations of success no doubt occur throughout the bird world. 

Geographic Variation. Geographic variations in the expected 
egg success clearly can be seen in the field. The American Robin 
of the mid-continent lays about four eggs and nests two and often 
three times a year. But the Robin of the Arctic lives in a land of short 
summers where one brood may be all that it can raise. In the one case, 
the pair may lay twelve eggs a year, in the other only four. Since 
the job of all eggs is the same replacement of the parents and birds 
dying without young the egg-success expectancy of the northern 
eggs seems to be three times that of the other. We do not know if 
there are differences in length of life or in life hazards which influ- 
ence this outcome. A low nest destruction in the Arctic and lower 
loss of fledglings may be one factor in the difference. Obviously, the 


low support rate of the Arctic with its low entrapment of biological 
energy makes a high breeding potential rather less necessary than 
farther south. 

Life Equations. Fig. 14-2 illustrates the principle of life equations, 
which simply means that the gains and losses from season to season 
balance each other. The number of birds rises sharply in the breeding 
season (with the advent of the young) to the annual peak; it declines 
throughout the year and reaches its low point at the start of the next 
breeding season. Comparable season numbers remain equated with 
those of other seasons. 

Life equations differ from the population equation (page 247) in 
that life equations concern the year to year gains and losses of a pop- 
ulation group. A life equation can be stated simply as follows: The 
number of birds of one year plus the additions of the breeding season 
minus the yearly losses equal the number of birds next year. 


Limiting Factors. The concept of limiting factors used in game 
management (Leopold, 1933) fits with peculiar aptness into thinking 
of bird populations. It applies well also to any bird attribute con- 
tingent upon ecological relations, such as bird abundance, spread, and 
distribution. In a broad sense, ecological limiting factors include con- 
cepts of the "minimum," such as the "law of the minimum" (Liebig's 
law), "law of the limiting factor," and "law of tolerance," though 
these have been elaborated more as principles of "organismal growth" 
than as population rules. 

Limiting factors are the nine intertwined ecological constituents 
of productivity that may be listed for convenience into two groups: 


Prcdation Competition 

Starvation Food and water supply 

Disease Climate 

Accidents |l Habitat 
x^ Special factors (dust baths, etc.) 

The limiting factor is the one most important at the time in re- 
stricting the population. A factor limiting at one time, season, or place 
may not be so under other circumstances. The absence of suitable 
mud for nest retorts (special factor) may be the limiting factor for 
the Cliff Swallow along a river canyon. Farther along the river, lack 
of suitable cliffs on which to attach the nests (habitat) may become 
the limiting factor. Regionally, climate may be the limiting factor, 
both with respect to population and to species distribution. Year to 


year variations in the limiting factors, however, may have less influ- 
ence upon population trends of some birds than their own density 
relations (Errington, 1945). 

Predation Relations. Predation and competition are components 
of environmental resistance which influence bird population in several 
ways. The predation influence upon the annual mortality of game 
species depends upon five variables (Leopold, 1933); these seem ap- 
propriate to bird species in general when expressed thus: 

Density of the bird population 

Density of the predator population 

Food preferences of the predators 

Physical condition of the bird and the escape facilities available to it 

Abundance of "buffers" or alternate foods for the predator 

Knowledge of the influence of predation upon populations is con- 
fused by the fact that it is only one of the agencies of environmental 
resistance. Birds vulnerable to predation may be the victims of one 
agency and by that action simply miss becoming the victims of an- 
other, so that many types of loss including loss from predation are 
at least inter compensatory in net population effect (Errington, 1946). 
Hence, the picture of wild bird populations rising and falling with 
increase or decrease of predation seems unreliable, although experi- 
mental populations of invertebrates in the confined quarters of a 
test tube may do so (Cause, 1934). In reality, predation is an energy 
transfer sequence of the food chain (page 443), and we may assume 
the validity of the empirical rule that no member of an energy transfer 
system destroys its subsistence base. It seems likely, though our 
knowledge is only fragmentary, that ordinarily population density 
relationships play a greater role than predation in controlling popula- 
tion levels. The variables influencing the population may thus be 
considered as either de^ity^e^end^t _^L.J^2l?i^d^^?^?M m 
character (Alice et al., rpfftjyf* 1 * 1 ^^ 

Predation studies indicate that the number of birds in excess of the 
carrying capacity of the habitat foiji a vulnerable group, at least in 
resident species. Pellet analysis, for example, showed Bob-white re- 
mains in 5.6 per cent of Horned Owl pellets when the prey were at 
36 per cent of the winter carrying capacity, and 19.0 per cent when 
insecure at 141 per cent of capacity (Errington, 1937). The preda- 
tion rate rose with increase of birds in excess of the number that the 
land could support adequately. 

Competition Relations. Competition may be the spice of life in 
human societies, but to birds it is a continual scramble from which 
only the successful emerge to survive or to reproduce. Fundamen- 


tally, birds compete directly with each other, though on occasion 
rivalry may be between bird and mammal or more rarely between bird 
and other organisms. It seems possible to list competition among birds 
under three headings: 

Interclass competition 

For example, between insectivorous birds and lizards for insects or be- 
tween Blue Jays and gray squirrels for acorns 
Interspecies competition 

For example, between hole-nesting birds for nesting sites, between sea 
birds for nesting rocks, or between birds in use of some food resources 
Intraspccies competition 

For example, contests over territory, intolerance at feeding stations, feed- 
ing upon the same food sources, or using the same cover 

Some animal ecologists have borrowed the term "coaction" from 
classical plant ecology; coaction thus used has as its principal phases 
competition, cooperation, and "disoperation" * 

Minimum Area and Minimum Numbers. Variations in the terri- 
torial area held by birds indicate rather clearly that a minimum size 
exists below which the birds fail to breed successfully. In addition to 
the minimum size of contiguous territories, there exists also a minimum 
size of isolated habitat that will be used by species. For the purpose 
of this principle of minimum area, isolated habitat will be considered 
as any habitat separated from others of its kind. An island may be iso- 
lated by the water around it or a forested butte by the surrounding 
desert or grassland. Yet equally isolated from the ecological stand- 
point may be a glade in the forest, a clump of white pines in the hard- 
woods, or a brush patch in farm fields. 

Minimum area and bird mobility are so interlinked that the daily 
cruising distance forms one of the limits to minimum area. But the 
better habitats have a rather lower minimum than poor ones. It may 
be presumed that where habitat is of choice quality, the bare minimum 
of area and cruising distance will have a chance to come into play. 
It has been said that the minimum isolated woodlot holding Ruffed 
Grouse is about 40 acres. An isolated quarter-acre marsh seems too 
small for the Song Sparrow and a woods of fewer than 5 acres too 
small for the Black-capped Chickadee. 

The absence of some birds from isolated habitats, particularly birds 
of low mobility, has been explained as possibly resulting from a "clean 
sweep" by local misfortune for the birds present. With no surround- 

* But a naturalist-philosopher expresses it better when he writes, "Nature red in 
tooth and claw that is, competitive, or nature at peace that is, cooperative; or nature 
in alternating moods, or at one and the same time, competitive and cooperative (her 
normal condition), is only in rare instances in balance for any extended period" 
(Bedichek, 1950). 


ing population to restock the area, permanent depopulation occurs 
(Leopold, 1933). Influx from nearby reservoirs repopulates an area, 
rapidly for mobile species, slowly for others. 

The limit in numbers below which a species may not go and re- 
cover has been termed the extwctio^tbreshold. This minimum seems 
to have been reached by the ill-fated Heath Hen when the number 
dropped to about 200. A million birds may have been the extinction 
threshold for the Passenger Pigeon. For birds like the Starling intro- 
duced into the United States, the limit is low, for only a handful sur- 
vived release to populate North America (see Figs. 10-11, 14-5). 

The influence of minimum numbers (when compared to known 
limitations of excess numbers) testifies to a principle of optimum num- 
bers, which can be stated simply as, "populations are most successful 
when in optimum numbers." The Passenger Pigeon nested in great 
concentration as well as in scattered groups; the latter were said to 
be less successful than the large ones. Large colonies of Herring and 
Lesser Black-backed Gulls show earlier laying, more uniformity, and 
greater success than others (Darling, 1938). Small colonies of Gulls, 
Terns, Gannets, and Fulmars have less success than large ones. A 
large colony of Yellow-headed Blackbirds has been reported to have 
a hatching success of 75.7 per cent in contrast to 60.6 per cent for a 
smaller one (Fautin, 1941). But few differences in success between 
large and small colonies of most species have been reported. A contri- 
bution to understanding the reasons for this difference in success ap- 
pears in banding records for the Common Tern colonies of Cape Cod; 
these records evidence the importance of longevity, site tenacity, and 
group adherence (Austin, 1945, 1949). 


Population Densities. Density carries with it the thought of bird 
numbers in terms of space (usually area) though in a stratified envi- 
ronment, such as a forest, it may be suitable to consider volume (page 
208). It can hardly be doubted that competition influences densities; 
yet in the year-around view, it can be assumed that basically the most 
important influence and determinant of bird numbers is the amount of 
food and cover present. It seems logical, though this has not been 
demonstrated as yet except in studies of a few gallinaceous game birds, 
that birds fill the environment to its capacity for them, the latter 
measured as carrying capacity (page 211). 

The density of bird population varies widely geographically and 
ecologically; the total bird population varies as well as that of the 
species. Any bird student who has passed through a succession of 


cover types at any season is aware of differences in abundance of birds, 
both the total population and that of the various species. Table 14-2 
shows some examples of comparative densities of birds in various habi- 
tats. The total species varied from twenty-one to forty-one and the 
density from 149 to 343 pairs per 100 acres. Table 14-2 also gives 
some parallel examples of comparative densities of the same species in 
the several habitats. 

Table 14-2 
Variation of Species Density in Various Habitats 




White Pine- 



Downy \Voodpecker 






Red-eyed Vireo 






Wood Thrush 





Tufted Titmouse 





Crested Flycatcher 





Song 1 Sparrow 









Density (pairs per 100 acres) 






In a study of winter birds of upland plant communities in North 
Carolina (Quay, 1947), the birds-per-acre densities were (November 
1, 1939 to March 1, 1940) as shown in Table 14*3. Similar variations, 
either by ecological stages, as in the table, or by other cover types, 
are found on every hand, irrespective of the methods of determination 

Mixed Densities. Because all birds must live from the same bio- 
logical energy resources, competition among them tends to adjust 
their respective densities. But if no overlap of their life needs occurs, 
the densities of the various species in a habitat are independent of each 
other. The Blue-headed Vireo feeding and nesting in the upper story 
of the forest has no overlap with the Ruffed Grouse on the forest 
floor; their respective densities vary independently of each other. 
But some overlap occurs, for example, in the feeding habits of Vireos 
and Warblers. When food becomes scarce, this may influence their 
respective numbers. Various Shorebirds feeding along the beach 
compete with each other whenever the same food attracts them. In 
the same way, birds that use the same space are competitive. The 
Mountain Bluebird and Violet-green Swallow, both of which nest in 
holes, compete with each other for nesting sites. 



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The net result of any joint use of habitat or any of its attributes 
is a compensatory adjustment of numbers. Mixed density actually 
means the density thus adjusted; it is not the same as the combined 
density of birds separate from each other or so interspersed through 
the habitat as not to influence the density of each other. It follows 
from this that the mixed density of two or more species is the sum of 
their normal, separate densities minus the proportion of overlap. Two 
species completely overlapping would have a mixed density identical 
with that of either alone. But if the overlap were 50 per cent, the 
mixed density would be of the order of three-fourths the combined 

While this discussion of mixed density stresses competition in the 
use of habitat, some "cooperation" may be taking place unseen. If the 
Blue-headed Vireo protects the tree against insect attack, there may be 
more buds available to the Ruffed Grouse next winter. That being 
the case, the mixed density will be higher than the sum of the respec- 
tive densities otherwise would be. Of this subject we know little. 

- Studies indicate that for many purposes at least, popu- 
lations^ sliould be considered on a mass rather than on a numerical 
basis. The total mass of animals measured by weight is termed the 
biomass, a most convenient term for the concept. Thus the number of 
Hairy and Downy Woodpeckers in a forest may be the same, but 
because the former is nearly three times the size of the latter (12 vs. 21 
grams), its population has nearly three times the biomass, though the 
numbers are equal (Fig. 10- 14)'. The habitat must support the birds, 
and our thinking is sounder when using size relations, for the biomass 
measures the avian tissue living off the energy resources of the land. 
Yet in all this, we must not overlook the fact that variations in bio- 
logical efficiency may interpose difficulties as yet immeasurable in our 
understanding of the biomass. It will be developed later in the section 
on food habits (Chapter 23) that birds vary in the amount of food 
needed. It has been shown also that the environment can support more 
biomass when organisms are in larger units than when in smaller ones. 
The famed flock of Passenger Pigeons estimated by Alexander 
Wilson to total 2,230,272,000 birds and to require 17,424,000 bushels 
of mast daily had a Pigeon biomass (340 grams a bird) of perhaps 
800,000 tons, which required about one-half million tons of mast 
daily (7 % ounces for each bird). Since fresh tree fruits like the acorn 
have about 54 calories per ounce (191 per 100 grams) of edible por- 
tion (about two-thirds of the acorn is edible), the daily consumption 
by the Pigeons thus estimated results in 360 calories per pound of bird. 
The Mourning Dove eats about 1 1 grams of grainaceous foods daily 
that average about 3.6 calories per gram. This would indicate a daily 


need for 39.6 calories, or about 180 calories per pound of Dove. The 
demands of avian biomass upon the habitat are indeed great, but the 
estimate by Alexander Wilson may have been twice as high as actu- 
ally the case for sustained daily consumption. (A "feast and famine" 
type of eating in the wild, however, is a very real thing.) 

The data of Table 14-2 have been converted, by using the probable 
average weights of the respective breeding birds, into biomass calcula- 
tions of grams per 100 acres of habitat for the several cover types 
given in Table 14-4. 

Table 14*4 
Data of Table 14*2 Expressed as Biomass 

^ ~, Biomass 

Cover Type , 

Oak-hickory .............................. 4,404 

Oak-maple ................................ 4,644 

Climax beech-maple ....................... 5,K91 

Lowland beech-maple .................... 7,023 

White pine-hemlock ....................... 8,H7 

In the oak-maple type, some 6,800 additional grams of biomass 
belonged to species feeding off the area. These uncounted birds (like 
Cowbirds and other visitors) must be kept in mind. The conversion 
shows differences that we may interpret roughly as support differences 
in the several habitats. The biomass-demity thus becomes a useful 
tool in population studies. Additional biomass data would be useful in 
furthering comparison. No doubt also, internal biomass comparisons, 
as of predator and prey, would prove useful. The total or habitat 
biomass could include all animal life in addition to birds; it also could 
be of the plant life or of the animal and plant life combined. 

**^ffi^^ QMMfr ' Because of the space needs of birds, 

whether terntonal or not, males unable to find suitable or unoccupied 
habitats will be unable to obtain mates (Chapter 12). Conversely, 
females unable to find males occupying habitat for nesting will like- 
wise go unmated. These two groups form a reservoir of unmated 
birds drawn upon to fill vacancies occurring among mated pairs. 
How large a reservoir exists is not known; indications are that it 
may become large, locally equalling the number of breeding birds. 

The unmated birds form a floating population, evidently not en- 
gaging in great strife with mated pairs but being tolerated by them. 
Records indicate quick replacement of one member of a pair lost 
through tragedy. A male Bald Eagle of a nesting pair was collected 
by an ornithologist. Within a week, he had been replaced by another 
that defended the nest as actively as his predecessor. Experimental re- 


moval of one of a pair of mated birds has shown replacement by 
another bird (possibly in late afternoon though more likely in early 
morning), so that the final parental duties were carried out by foster 
parents several substitutions removed from the original, biological 
parents. That there are differences in the remating rate of various 
species seems likely; likely also are variations with region, season, and 
breeding cycle. Differences in sex ratio and age classes may influence 
the utility of the reservoir. All of these points need investigation * and 
a few such studies have been made (e.g., Stewart and Aldrich, 1951). 

Population Cycles. The variations in bird numbers may con- 
veniently be considered as of an irregular Acyclic, or secular nature. 
Changes in the density of bird life resulting from environmental 
variation may be any one of the three, though our knowledge may 
not always permit a high degree of accuracy in determining which 
one. A change of bird life in the wake of a hurricane would be con- 
sidered as an irregular or random one, that occurring from a climatic 
cycle would be a cyclic one, and one in accordance with plant suc- 
cession would be a secular one. 

The number and variety of nonrhythmic fluctuations are almost 
unlimited, but rhythmic (periodic) ones are more limited in scope. 
Cycles of population abundance appear to be rather common, though 
very few have been analyzed. A long record of population usually 
shows cyclic fluctuation (perhaps always), random fluctuations, and 
a trend. The combined cyclic elements in such a record is termed 

Fig. 14*4. The logarithms of the European Partridge at Krumau as 
shown by game-bag records has a manifest cycle of about 23 years. This 
is composed of many periodicities of 'which the strongest cycle measures 
22.71 years. Irregular fluctuations and a trend are present also, but the 
latter has been neutralized by use of a 23 -year moving average. The lower 
curve consists of seven cycles derived from the Partridge record: 22.71, 
13.95, 11.84, 8.33, 8.05, 5.09, and 4.14 years. The shape and timing of 
these seven together 'will repeat identically only once in about five million 
years. (After Leonard W. Wing, "Cycles of European Partridge Abun- 
dance," Journal of Cycle Research, 2(1953):56-16.) 


a manifest cycle to distinguish it from the periodicities of which it 
may be formed or which may be derived from it. Fig. 14-4 shows a 
manifest cycle of abundance that proves to be composed of many 
periodicities, along with both random fluctuations and a trend. The 
intervals between highs and lows of such a manifest cycle may vary 
by several years, yet be composed of fixed cyclic elements that have 
become manifest in many cases for scores of years. 

The most common variations, both cyclic and noncyclic, are those 
of population number from year to year. One of the events involving 
both abundance and movement is the flight of northern birds, such as 
of the Snowy Owl which appears to follow a manifest cycle of about 
4 years in length. Population changes of the Ruffed Grouse have 
a manifest cycle commonly known as a "ten-year cycle." It is likely 
that any attribute environmentally related should be considered sub- 
ject to environmental fluctuation. In addition, internally controlled 
factors may show rhythms as an inherent trait of a biological system. 

Exotics in a New Environment. Two opposing views maintain 
respectively ( 1 ) that exotics cannot become established unless a vacant 
niche exists and (2) that a vigorous exotic species will carve out a 
niche for itself. It seems probable on theoretical grounds that no bird 
can survive in an alien climax. All exotics (at least so far as reliably 
known) established in strange lands are birds becoming established in 
settled areas (therefore disturbed) rather than in climax habitats; this 
fact supports the first view. 

The famed sigwoid population curve is used little in field orni- 
thology except in the study of exotics in new environment, expansion 
of resident birds under changed conditions, or population recovery 
from local misfortune. The sigmoid (s-shaped) curve reflects the 
characteristic of a population to expand slowly at first, then to rise 
rapidly, and finally to taper off at a new level (Fig. 14-5). 

All Starlings in America are quite likely descended from the 160 
birds (eighty pairs at most) liberated in New York City in 1890 and 
1891. The pioneer front appeared in an area first as winter stragglers 
mostly and perhaps always as young birds; establishment as a breed- 
ing bird followed after about 5 years or so. The Starling spread some- 
what more slowly than did the House Sparrow (Wing, 194 3 a). The 
Starling took more than 60 years to cover suitable area that the Spar- 
row overran within about 40 years of its introduction. The Starling 
migrates in America in a northeast to southwest direction. Its highest 
density extends as a belt from the Middle Atlantic states to about 
Central Texas. Washington, D. G, near one end and San Antonio 
near the other are both well known for the undesirable habits of 
Starlings. The Starling has also been introduced elsewhere and has 





1895 1905 

1915 1925 

1935 1945 

Fig. 14-5. The number of Starlings reported in the Christmas censuses 
per hour of censusing smoothed by moving averages of three and plotted 
on semilogarithm paper. Inset: The general shape of the sigmoid curve of 
population. (After David E. Davis, "The Growth of Starling, Sturnus 
vulgaris, Populations," Auk, 61(1950yA60-465.) 

thrived. It is the most widespread of thirteen European birds intro- 
duced into Australia and New Zealand (Williams, 1953). 

The British Goldfinch on Long Island, Mute Swan in the New 
York region and Oregon coast, European Tree Sparrow near St. 
Louis, Chinese Spotted Dove in Los Angeles, and Mynah in British 
Columbia have become established locally. Game birds introduced 
have been the Ring-necked, Chinese, and Mongolian Pheasants, the 
European Partridge, and the Chuckar Partridge. Native American 
birds which have been moved and therefore are exotic in some areas 
are the House Finch, Bob-white, Valley Quail, probably the Moun- 
tain Quail, and perhaps the Prairie Chicken. 


Relative Abundance of Species. Our information on bird popu- 
lation does not permit us to list relative abundance of species with 
complete confidence. If we may judge by the Christmas censuses, 


the most frequently reported and perhaps most abundant winter 
birds in eastern America are: 

American Crow House Sparrow 

Red-winged Blackbird Cowbird 

Starling American Goldfinch 

American Robin Black-capped Chickadee 

Slate-colored Junco Blue Jay 

Tree Sparrow Song Sparrow 

Cardinal Golden-crowned Kinglet 
Grackle (Bronzed and Purple) Tufted Titmouse 

Cedar Waxwing Mockingbird 

White-breasted Nuthatch Eastern Bluebird 

Horned Lark Eastern Meadowlark 

Downy Woodpecker Mourning Dove 

Our knowledge of bird abundance in summer does not give us the 
same assurance. The most abundant or most frequent ones over much 
of eastern North America seem to include: 

Song Sparrow House Sparrow 

Chipping Sparrow Cowbird 

American Robin Vesper Sparrow 

Mockingbird Yellow Warbler 

Red-winged Blackbird Barn Swallow 

Eastern Meadowlark Mourning Dove 

Total Populations. Just how many individuals of the various 
species or subspecies there may be is certainly an important objective 
for bird study to attain. Some indication of the bewildering variation 
in numbers of birds may be had from Table 14-5. 

Table 14*5 
Some Estimated Numbers for Several Species of Birds 

Species Number 

Whooping Crane 16-35 

Ross Goose 6,000 

Greater Snow Goose 25,000 

Laysan Teal 50 

White-tailed Ptarmigan 100,000 

Franklin Grouse 100,000 

White-winged Dove 5,000,000 

St. Kilda Wren 136 

Golden-cheeked Warbler 1,000 

Kirtland Warbler 1,000 

House Sparrow (America) 150,000,000 

Harris Sparrow 8,000,000 

Changes with Time. Important information has been obtained by 
observers that shows secular changes in bird populations, changes both 



for more and for fewer birds. On San Martin Island off the coast of 
Baja California, about thirty pairs of Ospreys nested in 1913; the 
population of 1946 consisted of only three pairs. Man is considered 
the most important enemy. The Common Redpoll has been rare in 
Ohio during the twentieth century, though often found there earlier; 
but Cardinals are more numerous than previously (Moseley, 1946). 

Fig. 14-6. Index of British Herons (1928, 1936, 1931, 1938, and 1939 
number of nests equals 100) compared to winter temperature. (After W. 
B. Alexander, "The Index of Heron Population, 1950," British Birds, 

Avocational British ornithologists have shown great zeal in obtain- 
ing an index (number of nests for 1928, 1936, 1937, 1938, and 1939 
taken as 100) of year-to-year Heron numbers. This shows that a most 
important influence is winter temperature. After an unusually cold 
winter, as that of 1946-47, recovery of normal numbers requires 2 
to 4 years (Fig. 14-6). 

A study of interest records the growth of a Caspian Tern colony 
in the San Francisco Bay region from 1922 to 1931 (Miller, 1943). 
The yearly count from 1922 to 1931 is given in Table 14-6 (after 
a lapse of 12 years in the count, 378 nests were found in 1943). A 
5-year census of a virgin Palouse Prairie (Wing, 1949) showed a 


Table 14-6 
Caspian Tern Count of a San Francisco Bay Colony 


N " b , er Year 
of Birds 

of Birds 


7 1927 



2 1928 



... 12 1929 

1925 (estimate) 

. 35-50 1930 



164 1931 


Source: Alden H. Miller, "Census of a Colony of Caspian Terns," Condor, 
45(1943) -.220-225. 

relative uniformity of total numbers per hundred acres, though some 
intercompensatory adjustments occurred (Table 14*7). Counts of 
Common and Roseate Terns on Weepecket Island, Massachusetts, 
varied between 2,000 and 4,000 from 1896 to 1940. They were dis- 
placed by Herring Gulls, which varied in number from four in 1935 
to 1,000 in 1943 (Crowell and Crowell, 1946). Actually, examples 
of changes in bird life may be found throughout the world; some may 
be attributed to man but others may not (Fig. 14 7). 

Table 14-7 
Breeding Bird Censuses on a Virgin Palouse Prairie 


Year of Birds 

1942 248 

1944 227 

1945 258 

1946 248 

1947 248 

Average 246 

Source: Leonard W. Wing, "Breeding Birds of Virgin Palouse Prairie," Auk, 
66(1949) :38-41. 

Total Number of Birds. Because we have no data on the bird 
population of America in pre-Columbian times, our conclusions on 
changes with settlement must be based largely on knowledge of cir- 
cumstances. As mentioned elsewhere, some birds have declined in 
numbers, some have increased. But because the climax appears to be 
the most efficient energy stage (page 199), it seems likely that it in 
turn supports a higher population of birds, at least on a biomass basis, 
than other ecological stages. Censuses of birds in climax and non- 
climax environments support this assumption, though the many vari- 



Fig. 14 7. Invasions of the Black-necked Grebe into Europe in the 
twentieth century are thought to be associated with drought conditions 
in the dry lands of Central Asia. (After Olavi Kalela, "Changes in Geo- 
graphic Ranges in the Avifauna of Northern and Central Europe in Re- 
lation to Recent Changes in Climate," Bird-Banding, 20(1 949) :10-103.) 

ables involved make comparative evaluation difficult. On theoretical 
grounds at least, it seems entirely probable that compared to 1492 both 
fewer birds and a smaller avian biomass exist today in North America. 
The total number of birds today may be estimated for a few areas 
on the basis of census reports, such as the Christmas censuses or 
breeding bird censuses of the United States and Canada. In addition, 
sample censuses reported by various observers add to our information. 
Several attempts have been made to form a suggestive estimate, and 


no doubt more and improved ones will be forthcoming from time to 
time. On the basis of our present knowledge, the breeding populations 
of the United States seem to be about 5.6 billion birds. A comparison 
between appropriate climax and nonclimax areas indicates that the 
former runs substantially higher in bird life, sometimes more than 25 
per cent greater. This would perhaps permit us the assumption that 
the bird life under pristine conditions totaled proportionately more. 
On the basis of kinds of area in the much smaller space of Great 
Britain, hence one where studies are far easier and the problem less 
complex, the bird population is estimated at about 120 millions 
(Fisher, 1940). The world's bird life may in turn total one hundred 


SCHMIDT, Principles of Animal Ecology. Philadelphia: W. B. Saunders Co., 1949. 

CLEMENTS, F. C., and VICTOR SHELFORD, Bio-ecology. New York: John Wiley & 
Sons, Inc., 1939. 

ERRINGTON, PAUL L., "On the Analysis of Productivity in Populations of Higher Ver- 
tebrates," Journal of Wildlife Management, 6 (1942): 165-181. 

ERRINGTON, PAUL L., "Predation and Vertebrate Populations," Quarterly Review of 
Biology, 21 (1946): 144-177, 221-245. 

KENDEIGH, S. CHARLES, Measurement of Bird Populations. Ecological Monographs, 
14(1944) :67-106. 

KLUIJER, H. N, "The Population Ecology of the Great Tit," Ardea, 39(1950:1-135. 
"LEOPOLD, ALDO, Game Management. New York: Charles Scribner's Sons, 1933. 

LOTKA, ALFRED J., Elements of Physical Biology. Baltimore: Williams & Wilkins 
Co., 1923. 

PEARL, RAYMOND, The Natural History of Populations. New York: Oxford Uni- 
versity Press, 1939. 

PEARSE, A. S., Animal Ecology. New York: McGraw-Hill Book Co., Inc., 1939. 

SIMPSON, GEORGE GAYLORD, and ANNE ROE, Quantitative Zoology. New York: 

McGraw-Hill Book Co., Inc., 1939. 

*WiNG, LEONARD W., Practice of Wildlife Conservation. New York: John Wiley & 
Sons, Inc., 1951. 


Bird Flight 

As a means of moving under low power, swimming has advantages 
over land and air travel. The ease with which a man can push a 
water-borne burden or the low horsepower per ton that moves a 
loaded boat illustrates these advantages as compared to land or air 
travel. Additionally, the animal does not have to use energy for carry- 
ing the body or maintaining posture; the water supports the weight. 
But water travel is a slow-speed proposition. Increase of speed in 
water means a very great increase in resistance. It also confines the 
water animal to water areas. The ability to travel on land, however, 
opened new horizons to the ancestral land animals previously confined 
to the sea. It also opened opportunities for a little more speed and 
mobility, though this increase was probably not very great. 

In like manner, moving a burden by air is more costly in consump- 
tion of energy than moving it by land or sea. The bird in flight must 
carry the weight by support generated with muscle rather than rest- 
ing it on the ground or supporting it in the water. But greater speed 
is possible and especially so is greater mobility; the flying bird moves 
into a realm having few occupants. The cost in efficiency is a 
rather small price to pay for the great advantages offered by flight. 

The ability to fly and all the many advantages that consequent 
mobility and speed confer upon its possessors dominate the life of the 
bird. Even though we can study the facts of flight, many of which 
have been clarified by man's learning to use mechanical flight, our 
knowledge of its origin must be largely conjectural, but knowledge 
of the facts of flight does make conjecture intelligent. 


Four or five theories have been advanced to explain the origin of 
flight; several additional ones concern its early development (as dis- 




Fig. 15-1. Drawings to illustrate some theories of the origin of flight: 

(a) Arboreal annual with flight feathers developing on limbs and tail. 

(b) Cursorial animal with feathers developing on forelimbs for beating 
the air. (c) Four-winged stage of gliding arboreal animal, (d) Arboreal 
annual with feathers developing on forelimbs, side, and tail for gliding. 

tinguished from its origin). The theories of origin rest upon certain 
facts. The flight of birds, for example, involves the forelimbs, with 
no primary adaptations of the hindlimbs for flight. The motive power 
has been transferred from the forelimbs to the body (pectoral) 
muscles. No living or fossil bird shows development of any flight 
devices other than forearm wings (except possibly the postulated 
four- winged stage), and most (if not all) structures associated with 
flight are but modifications of standard vertebrate possessions. Bird 
flight, be it noted, makes effective use of the various known principles 
of, aerodynamics. 

Cursorial Origin. The cursorial theory (Nopsca, 1907, 1923) 
holds that birds arose from a running ancestor that sped over the 
ground on its hindlimbs, its forelimbs flailing the air to aid balance. 
The theory proposes further that growth of scales which flattened 
into feathers aided their wearer in increasing its stride into a series 
of leaps lengthened by the flapping of these "feathered" forearms. 
In time, this developed into -flight (Fig. 15'1). "Water- walking" 
maj^perhaps have been somewhat similar (Erickson, 1955). 

Arboreal Origin. The arboreal theory has the flying bird de- 
scended from an ancestor that climbed trees to live an arboreal life 


(though the arboreal ancestor itself may have evolved from a ground 
ancestor). The claws and fingers on the wings of Archaeopteryx 
combined with two wings having weak musculature suggest to the 
proponents of the arboreal theory a soaring or gliding animal. Thus, 
the animal climbed about in the trees and glided or volplaned to an- 
other necessarily lower position, somewhat as do the flying squirrel 
and flying lemur of today (Fig. 15-1). 

The discovery of apparent vestiges of quill feathers on the thighs 
of a number of embryos (and possibly upon adults also) as well as 
upon Archaeopteryx has given rise to the postulate that an early stage 
in flight involved planelike feathered structures on both the fore- and 
the hindlimbs to give a four- winged condition (Beebe, 1915). The 
forelimbs dominated so that the four-winged condition went out of 
style (Fig. 15-1). 

^Combination Theory. The ideas of arboreal and cursorial origins 
haveHBeen combined into one holding that the pro-aves animals both 
ran on the ground and ascended trees, where they perched upright on 
limbs. The hindlimbs were somewhat like those of birds of today and 
bipedal dinosaurs of yore. The three fingers were for climbing, and 
the animals hopped lightly from limb to limb, partially supported in 
transit by folds of skin at the joints of arms and legs. Later, long scale- 
feathers developed on the forelimbs, hindlimbs, and tail. Flapping 
the forelimbs bearing these scale-feathers provided an advantage 
which in time gave rise to wings. 

Diving (5rigin. On the assumption that water birds are the most 
primitive birds today and arboreal ones the most specialized, the 
diving-origin theory suggests that flight arose from gliding or soaring 
out over the water for fish, the pro-aves starting from a cliff or elevated 
perch and swimming back to land. The strong-flying sea birds arose 
thus from these ancestors and the land birds from sea birds. While 
this has been proposed as an additional theory, it really does little 
except to substitute Cliffs for trees in the arboreal theory. 

Wind-Response Origin. Organisms meet the problem of wind in 
the environment by passiveness on the one hand (as by loss of flight 
or by hiding low in the grass) and by aggressiveness on the other (as 
by greater flight powers or by strong climbing) . In aggressively meet- 
ing the wind, the wind-response theory proposes that the pro-aves 
animal developed tendencies to let wind carry some weight; the 
strength thus released was used for locomotion on the ground; pene- 
tration of the trees and water came later. The forelimbs did not 
decline in vigor to reverse their evolution later in becoming wings, as 
bipedal theories usually must assume. 



Fishes. Fishes of several families in warm marine waters (and a 
few of tropical fresh water) have developed aerial locomotion of sorts. 
But none uses its "wings" (actually enlarged pectoral fins) to flap or 
otherwise to propel itself through the air. Some members of the 
family Cypseluridae (perhaps all) "fly" by unfolding the pectoral 
fins and gliding stiff finned. They swim at high speed immediately 
under the surface, rise to the surface, spread out their fins and taxi 
along, propelled by vigorous side-to-side motions of the especially 
powered tail, almost always into the wind (Schultz and Stern, 1948). 
If sufficient lift is generated, the fish may leave the water entirely 
for from 2 to 20 seconds and for 10 to 60 feet after a run of about the 
same length in the water (Fig. 15-2). Flights lasting more than half 
a minute and reaching more than 30 feet above the water and 1,000 
feet long have been reported. The speed of taxiing and flight may 
reach 20 to 40 miles an hour. It should be noted that this differs from 
true flight in that true flight always receives its generating or con- 
tinuing force in the air. 

Amphibians. The flying frog (Polypedates) of the East Indies 
makes a slanting glide, reported to total not more than 30 to 40 feet, 
from an elevated position. These frogs have large webbings stretched 
between the toes of all feet (Fig. 15-3). 

Reptiles. Pterosaurs, often called "flying dragons," have been the 
reptilian bid to dominate the air (Fig. 15-3), though none survived 
the Mesozoic era. It seems reasonably clear that the pterosaurs 
descended from thecodonts. Presumably, they had a batlike locomo- 
tion; perhaps they also clung to objects like bats. 

The fifth (commonly called "little finger") of pterosaurs grew long 
and strong to stretch a membrane for flight (see Fig. 1-14). The 
remaining fingers remained small, though still clawed. If we may judge 
by the bones for attachment of flight muscles, the larger pterosaurs 
had a weak flight at best, possibly a soaring or Vulture-like flight. 
The smaller pterosaurs may have been somewhat batlike in flight. 

Flying reptiles varied in size but were comparable JQ modern birds. 
The largest (Pterario(fohy'bwn&~a~25-Tooi wing spread, the greatest 
of any animal living or dead. Perhaps it soared and occupied a niche 
in the Mesozoic community somewhat as Vultures do today; we may 
assume safely that fleshy animals died and provided food for any 
Vulturine pterosaur. The horny beak of Pteranodon and the weak 
teeth of Rhamphorhyncus suggest the habit of feeding upon inactive 





food. But one can hardly postulate a use for the great bony crest on 
the head of Pteranodon (Fig. 15-3). 

A small, living lizard (Draco volcrns) of southeastern Asia and ad- 
joining islands has a thin membranous skin over extended ribs to 
form a gliding surface. The lizard can glide from a high to a lower 
position. Writers sometimes called it a "flying dragon." 

Fig. 15 3. Flight in the vertebrate 'world other than in birds, (a) "Fly- 
ing Frog" (b) Pterodactyl, (c) Flying Squirrel, (d) "Flying Lizard" 
(e) Bat, (f) Pteranodon. 

Mammals. The only mammals to develop the power of flight be- 
long to a single order, Chjmjtpra (Fig. 15-3). Flying squirrels and 
flying lemurs actually gliderrom a high to a low elevation by a 
peculiar looseness of the body skin (Fig. 15-3). All bats fly by means 
of a membrane stretched between the fore- and hindlimbs, sometimes 
also including the tail (see Fig. 1-14). The elongated fingers of the 
bats support the membrane, but it is the third or middle finger that is 
elongated most; the first (thumb) remains free. The flight muscles of 
bats have considerable power and in consequence the breast bone pos- 
sesses a well-defined keel, an evolutionary parallel of the bird. 

Bats lack the aerial powers of birds, however, which fact results 
from several less advantageous characteristics. The feathers of the 
bird shape the wing to make a highly efficient airfoil (Fig. 15-4). The 
structure of the feather makes possible such characters also as "slots," 
"flaps," "propellers," and the like, in accord with sound aerodynamic 


principles. The bird has become modified far more for an aerial life; 
the bat itself differs little anatomically from a land mammal except for 
wings. Its flight muscles average about 7 per cent of the body weight 
in comparison with the average of "17 per cent for the bird (Hankin, 
1913). In addition, the bird has a superior streamlined body shape. 
Bats with their supersonic detection device have a very great, spe- 
cialized adaptation for a particular type of night hunting For insects. 


Air movement 

Fig. 1 5 '4. Left. Relationship of lift and drag to an airfoil at a moderate 
angle of attack. The major lift occurs in the part of the wing marked by 
the dotted line. Right. The airfoil of a Vulture (Otogyps calvus). Air 
passing over the top of the wing must travel faster than that passing across 
the lower surface. Hence, the air pressure is lower on the top so that the 
wing exerts lift. This is in accord with Bernoulli's theorem (as applied to 
unc on fined gases). 

But cruising does not seem so efficient a use of energy in hunting 
insects as does the method of the Kingbird, for example, which stays 
at rest and thereby saves energy until insects come by. Yet the bat 
can utilize a food source otherwise largely unexploited by the verte- 
brate world. Its night life follows the tendency of small mammals to 
be nocturnal, an uncommon tendency among birds. 


Flight Structure. The advent of the airplane has clarified many 
points about bird flight while obscuring others (though no aerody- 
namic principle used by airplanes appears to be unused by birds). 
Body modifications have been described (Chapters 3, 4, 5), though 
they are so many and so varied that they cannot be given justice in 
a treatment covering much less than a whole volume in itself. The 
fundamental structures of flight are ( 1 ) the skeleton that forms the 
framework, (2) the muscles that propel the wings, (3) the nervous 
tissue that coordinates action, (4) and the feathers that meet the air 


The skeletal parts are rigid in construction and light in weight. 
The Frigate or Man-o'-war Bird, for example, weighs about 32 ounces 
and has a wing spread of 7 feet; yet its bones are said to weigh about 
4 ounces. Some other skeletal and body weights are given in Table 
15*1. The shoulder joint has relatively great freedom of movement. 

Table 15*1 
Body and Skeletal Weights 


Body Weight 

Skeletal Weight 

Canada Goose 



Blue-winged Teal 



Golden Fagle 

. 4,750 


Screech Owl 



Barred Owl 



American Robin 






The elbow moves forward and backward only; its rigid construction 
precludes up and down motion. The hand has considerable movement 
at the wrist joint but little otherwise except for the opening and 
closing action of the alula. The wrist joint retains sufficient flexibility 
to open, spread, and rotate for advantageously opposing air currents. 

The primaries, but especially the secondaries and sometimes ter- 
tiaries, serve as the supporting surface opposed to the air. The warm 
air in the body and feathers decreases specific gravity slightly. The 
wing coverts function in flight almost entirely to shape the wing in 
forming an airfoil of efficient design (Fig. 15-4). (Airfoil is a term 
of aerodynamics meaning the shape of a cross-section of the wing; 
though in England and sometimes elsewhere, it may also be used inter- 
changeably with iving.) They serve also to give a smooth surface 
having low friction with air. Because all feathers overlap each other, 
a rather solid, airtight structure results. 

While all surfaces may serve to supply lift, it comes chiefly from 
the inner wing (secondaries and tertiaries), primaries, and tail, in that 
order. From time to time, it may be noted, relative lift by the sup- 
porting structures may vary somewhat from the usual condition. The 
primaries are said to function principally as producers of forward 
motion, similar in this respect to the propeller of an airplane. But this 
interpretation has been questioned by aerophysicists. The alula func- 
tions as an air slot to increase lift; it may possibly serve"a33Sronally 
as a "paravane" in diving birds. The tail serves as an extra planing 
surface, though at times it may act as a slot, flap, rudder, elevator, or 


Properties of Air. Air, like other gases, varies in density with 
temperature and composition. Dry air at standard conditions (32 F. 
at 29.92 inches of barometric pressure) weighs about 0.081 pounds 
per cubic foot. Pressure and temperature of air change with altitude, 
as any mountain visitor can testify. (For ordinary field work, we may 
consider that temperature drops about 3 F. for each 1,000 feet rise 
in altitude.) Birds live only in the lower atmosphere, even on moun- 
tain tops. 

Because water vapor has a density but about five-eighths that of air, 
humidity lowers lift and increases the power needed. In the same way, 
the less dense air of high altitudes offers less lift. In consequence of 
lowered densities in humid atmospheres and high altitudes, some ad- 
justment of wing loading occurs among birds adapted for high alti- 
tude and for humid regions. 

Flight Mechanics. Air resists the efforts of a body to move 
through it; conversely, moving air has force. From the standpoint of 
an object, the force is the same whether the air moves and the object 
remains stationary or the object moves and the air remains motion- 
less. Flight concerns itself with the relative movement of the air and 
object. Air flowing around a perfectly streamlined object would exert 
pressure equally on all sides. But transfer of force to the object will 
move it when the force exceeds the inertia or anchorage of the object. 
This is the plan of the wing (Fig. 15-4). The greatest reduction of 
pressure occurs at the top and especially at the leading edge where 
the wing first meets the passing air stream. Hence, the wing supports 
the bird. If the angle of attack becomes too great, the leading edge 
"stands in its own air-stream shadow." The streamline of air does not 
follow the air foil; lift is lost. To prevent stalling or to lower the 
stalling speed of the moving air-borne body, slots are used to deflect 
or to pass the air stream back along the upper wing surface. In a 
sense, the slot acts like a small airfoil ahead of the leading edge. The 
bird uses the alula to deflect the air current down to the wing at slow 
* speeds with high attack angles. Slots may also be formed between 
the wing and body to smooth the air stream over the tail. It is prob- 
able also that the primaries may at times form air slots, particularly 
the outer ones with their greater flexibility and control. The trailing 
edge of the wing cannot be let down like an airplane flap to increase 
camber and lift at slow speeds, but the wing may be rotated to increase 
camber. The tail may be used as a "flap" also. 

Because of nerve endings in the skin near their bases, nearly all 
feathers of the wing can act as sensory receptors. Practically every 
point of the wing may respond and thereby bring about continuous 
variations and adjustments up and down the wing, all of which make 


difficult the task of comparing bird wing action with that of a fixed, 
mechanical wing. 

The primaries twist to take advantage of air conditions, reportedly 
somewhat asjaiay a variable-pitch propeller in an airplane. The outer- 
most primary with the superior control encumbent upon it is evidently 
the most important feather. Indeed, it is reported that a few species 
may be nearly or quite flightless if this primary is lost. But the concept 
of wing-tip propellers is a postulate probably not based upon sound 

The slight difference in pressure on the upper side of the wing 
exerts very great lifting force. An average difference over the wing 
surface (280-295 square inches) of a Barred Owl (weight 600- 
900 grams) amounting to but one-hundredth of a pound per square 
inch would give a total lifting power of nearly 3 pounds, which would 
carry the bird even with a rat in its talons. 

In general, lift and drag vary with square of the speed. Thejjft 
varies more or less proportionally to* angle of attack and area of wing. 
Twice the wing area or twice the angle would carry twice the load 
but to double the speed would take eight times the power. Twice the 
camber (curvature), however, would about quadruple the drag. 

The ratio of length to width of the wing is called the aspect ratio, 
and is large for soaring birds that live in the open and ride the air cur- 
rents. Because tip vortexes cause reduction in efficiency for some dis- 
tance inward, a long wing increases efficiency for flyers by increasing 
the proportion of inner, more efficient wing. For birds that need a 
very great power most of the time or that must operate in close quar- 
ters, the wide wing of small ratio gains more in usefulness than it loses 
in efficiency. 

A moving body sometimes generates static electricity, which pre- 
sumably occurs in birds also; at least it can be done experimentally 
with feathers. Whether the factor of static electricity influences the 
flying bird is not known. 

Wing Loading. Birds tend to have variable wing loads (weight 
per unit of wing) in accordance with species and flight pattern. 
Soaring birds have low wing loads; fast flyers with rapid wing beat 
have larger ones. No doubt the additional bearing surface brought 
against the air when need be, such as the tail, enters into the loading. 
Aleasurements of the Barred Owl and Ruby-throated Hummingbird 
are given in Table 15-2. 

Wing loadings, in pounds per square foot, without allowing for 
lift from the tail or for differences in lift between the secondaries and 
primaries, are given in Table 15-3. Wing loadings, it should be noted, 
do not take into account the length or efficiency of various wings. 



The wing load in pounds per square foot of some soaring birds has 
been reported as (Hankin, 1913): Adjutant, 1.54; Cheel, 0.55; White 
Vulture, 0.87; Common Vulture 1.13; Old World Black Vulture 
(Otogyps), 1.23. 

Table 15*2 

Measurements of Barred Owl and Ruby-throated Hummingbird Expanse 
(Square Centimeters) 

Barred Owl 
(Sq. Cm.) 

(Sq. Cm.) 
















Table 15*3 
Wing Loading 



Wing Area 
(Sq. Cm.) 

Pounds per 

Grams per 

Black-capped Chickadee . . 
Barn Swallow , 

12.5 . 




Tree Swallow 


House Sparrow 


Leach Petrel 


Purple Martin 




American Robin 


American Sparrow Falcon . 
American Woodcock 




American Crow 


American Marsh Harrier . . 
Peregrine Falcon (male) . . 
Peregrine Falcon (female) . 
Mallard Duck (female) 

, . . 1,222 

Mallard Duck (male) 

. . 1,408 


. . 1,798 

Turkey Vulture 


Golden Eagle 

. . 4,664 

\Vhistling Swan 

. . 5,943 

Source: Adapted from Earl L. Poole, "Relative Wing Weights of Bats and Birds," 
Journal of Mammalogy, 17(1936):412-443, and Earl L. Poole, "Weights and Wing 
Areas in North American Birds," Auk, 55(1938):511-517. 


Maneuvering. Birdsturn and ^ twist m th^air. 
the relative pressure against the respective wings. While all birds 
(including Swifts) beat their wings in unison, they can raise, lower, 
shorten, twist, and even stop one wing while the other continues to 
suppljT power. One wing can act as a brake to turn the bird. In some 
cases, the tail may be brought into play to serve as a rudder, but more 
often the tail is used to "trim" the flight. 

Birds alighting upon a solid substance must stop their forward 
motion in time to prevent damage to their relatively weak legs and 
feet as well as to the body itself. This they accomplish by increasing 
the angle of attack, spreading the wings, spreading the tail downward 
like a flap, back paddling, and sometimes by elevating the body proper 
against the air stream. 

Water birds do not need to use as much care in braking to a stop 
before alighting, because the water itself will cushion the impact some- 
what. Some diving birds close their wings and plummet to the water, 
depending upon the force of their fall to carry them down. But others 
increase the angle of attack to a stall and go down by a tailspin-like 
plunge (page 440). Plungers may have the body cushioned for sup- 
plementing the protection afforded by the feathers (see Fig. 23 3). 

Wing-Flapping Rate. The flapping rate depends upon flight style, 
size and shape of wing, motion rate of the wing pressure center, 
ground speed, and air motion (Blake, 1947). That it varies with the 
intention of the bird as well as with aerodynamics seems obvious, just 
as the stepping rate of a vigorous man depends upon where he is 
going uphill, downhill, or wherever and on how rough is the 

Table 15*4 
Wing-Flapping Rates 

c . Rate . Rate 

S P ecies per Second S P ecies per Second 

Double-crested Cormorant ..... 2.6 Tree Swallow ................ 3.5 

Black Duck (?) .............. 2.0 Bank Swallow ................ 2.8 

American Sparrow Falcon ..... 2.4 Roii^n-winged Swallow ....... 3.9 

Ring-necked Pheasant ........ 3.2 Barn Swallow ................ 3.9 

Killdeer ..................... 2.4 Cliff Swallow ................ 3.9 

Great Black-backed Gull ...... 2.0 Purple Martin ................ 4.4 

Herring Gull ................ 2.3 Blue Jay ................... 2.6, 3.4 

Laughing Gull ................ 2.45 American Crow .............. 2.0 

Rock Dove .................. 3.0 American Robin .............. 2.3 

Mourning Dove .............. 2.45 Eastern Bluebird ............. 3.1 

Belted Kingfisher ............. 2.4 Starling ..................... 3.3 

Northern Flicker ............. 2.2 Eastern Goldfinch ............ 4.9 

Source: Charles H. Blake, "Wing Flapping Rates of Birds," Auk, 64(1947):619-620, 
and Charles H, Blake, "The Flight of Swallows," Auk, 64 (1948): 54-62. 


footing and how fast he wishes to go. It would vary also according 
to the load carried. 

The flapping rate of birds is coordinated with the shedding of 
vortexes from the trailing edge of the wing; these are shed alternately 
up and down. Many differences in flapping rates occur among species, 
but the human eye has difficulty counting more than seven or eight 
flaps per second, so that those for small birds or fast flyers are difficult 
to obtain. Some average wing-flapping rates per second are give'n in 
Table 15-4. 

That variations occur with the kind of flying is shown in the aver- 
ages given in Table 15-5. 

Table 15*5 
Variation of Wing-Flapping Rate with Flight 

Species Rare 

and per 

Type of Flight Second 

Blue Jay 

Takeoff 4.0 

Steady flight 2.6 

Flap-glide 2.2 

Bank Swallow 

Coursing 2.8 

Climbing 3.7 

Cliff Swallow 

Coursing 3.9 

Quick-napping 4.6 

Source: Charles H. Blake, "Wing Flapping Rates of Birds," Auk, 64 (1947): 61 9-620; 
and Charles H. Blake, "The Flight of Swallows," Auk, 65 (1948): 54-62. 


For convenience and largely in reality also, bird flight may be 
divided into (1) poiver flying, (2) gliding, (3) soaring, and (4) 
special flying. The ordinary flight of birds can be considered as 
power flying, though great variation occurs. Some flight patterns 
merge or overlap others. 

Power Flying. The wings move up and down but with enough 
additional direction of movement during power flying to propel the 
bird. The downward sweep on a power flap usually has a forward 
thrust, with the primaries twisted at an angle to the motion. The 
resistance of the air against the feathers puUs..the bird downward and 
forward, the power of the wing against the .air coming from the great 
outer breast muscles. But the secondaries also act against the resisting 


Table 15-6 
Comparison of Depressor and Elevator Muscles of Breast 



Depressor Muscles 

Flcvator Muscles 

., . Per Cent 

M U .*? of Body 
Weight Wcigh > 

, . , Per Cent 
'\ USCC <>f Body 
Wugl* Wcigh 7 t 

American Robin 72.5 
Ruby-throated Hum- 
mingbird 2 .40 

10.01 13.90 
0.51 20.5 

1.15 1.60 
0.23 9.25 

Source: D. B. O. Savile, "The Flight Mechanism of Swifts and Hummingbirds," 
Auk, 67(1950):499-504. 

air to balance the downward pull of the primaries with the result that 
the bird moves horizontally, usually with no loss of altitude. The 
upward stroke receives its power from the inner breast muscles, which 
are smaller than the outer ones. The weights of breast muscles of the 
American Robin and Ruby-throated Hummingbird illustrate this 
(Table 15-6). The latter hovers before a flower and must call upon 
the elevator muscles to help maintain the bird in the air, whicn it must 
do without much aid from the air stream, meanwhile counterbalancing 
the action of the depressor muscles (Savile, 1950). The Swift has a 
rather similar wing (Fig. 15-5). 

The rotation of the outer primaries and adjustment of the sec- 
ondaries together during flight give a horizontal thrust for forward 
movement. Yet despite the fact that the secondaries move up and 
down with the wing, they give support at all times, whether going 
up or down. Relative air pressure on the upper and lower surfaces 
determines the support irrespective of wing direction, though the dif- 


Fig. 15* 5. Whig-shape of (a) Ruby-throated Hummingbird and (b) 
Chimney Swift; (c) probable wing action of Swifts and Hummingbirds 
in level flight during down stroke; (d) same during up stroke. (After 
D. B. O. Savile, "The Flight Mechanism of Swifts and Hummingbirds, 
Auk, 61(1950):499-504.) 



f erence can be greater on the down stroke. Hence, the wing supports 
the body more efficiently on the down stroke. 

Gliding and Soaring. Gliding and soaring flight differ little ex- 
cept that we consider gliding to be from a higher to a lower altitude 
("sliding down hill") and soaring as flight by riding rising air cur- 
rents. Any flying bird seems able to glide if the angle of coasting is 
steep enough. Thus the American Robin or Mockingbird can glide 
from a perch down to the ground on set wings. Even the Grouse, 
which have no soaring ability, can glide from a ridge to the valley 
floor. (The hunter often calls it "scaling.") 

Wind direction 

Fig. 1 5 6. The path of a soaring Haivk. The arrows show direction of 
wind and bird, respectively. Note that the bird moves forward more 
slowly into the wind than with it. 

On sunny days, air rises over warmer surfaces, and sinks over 
cooler ones. These currents of air (called thermal*) provide means by 
which Vultures soar on clear days, in a sense by "sliding down hill" on 
rising air. They can rise or sink according to the power of the rising 
air and the advantage taken of it (Fig. 15-6). Warm air rises and 
cold air sinks, which cause vertical air currents. Rising air currents 
occur also when moving air strikes any obstruction that deflects it up- 
ward. Birds soar along topographic features (ridges, cliffs, shores) 
that deflect upward the passing horizontal air streams. 

How capable the bird may be in soaring depends upon its ability 
to coordinate its movements as well as its airworthiness. The minimum 
angle at which a bird can glide depends upon .its sinking speed, which 
in turn depends upon the wing loading (Kuchemann and von Hoist, 
1941). The minimum sinking speed of the Turkey Vulture is calcu- 
lated to be 2 feet per second (1.36 miles per hour) and that for the 
Black Vulture 2.6 feet per second (1.77 miles per hour) (Raspet, 
1950). The maximum lift coefficients of the two birds have been 
determined to be 1.60 and 1.57, respectively. Just how much power 


a bird requires to maintain itself in the air, that is, to equal its minimum 
sinking speed, should be of considerable interest. For the Turkey 
Vulture, the sinking speed indicates that 0.017 horsepower is needed 
to keep one weighing 5.5 pounds in the air. The Black Vulture with 
its higher sinking speed will require somewhat more, which calculates 
to be 0.019 horsepower for 5.0 pounds. Because power-producing 
muscle can generate for some time an output of about 1 horsepower 
per hundred pounds, the Vultures seem adequately powered for their 
type of flight. In power per pound of weight, they are far more effi- 
cient than any power plane devised by man and matched at best only 
by a high-performance sailplane. 

Sea birds also soar in updrafts of thermals, but thennals usually arc 
less abundant over water than over land. Many birds follow the air 
currents rising over waves, which can be noted by watching birds soar 
along crests just above the water. Cold air over warm water, as in 
winter, results in convection currents (though heating of air by the 
water is low). Hence, these convection currents over oceans are 
strongest in winter and seasonal variations may account for seasonal 
shifts of pelagic birds (Woodcock, 1942). Other sources of rising air 
over oceans occur around islands and rocks; the fact that Gulls ride 
the air currents around a steamer has long been known. 

A marked difference occurs in the soaring mechanics of land and 
sea birds that probably has special significance. Land-soaring birds 
spread the outer primaries to form distinct slots, which are absent in 
soaring sea birds. Presumably the less stable air currents over land 
make the slotted wings necessary or advantageous. But in gliding, the 
Vulture closes its wing slots, which thereby lowers the drag coeffi- 
cient from a minimum of 0.19 to 0.0058 (Raspet, 1950). Soaring 
efficiency for this species presumably then approaches more nearly 
that of some soaring birds of the sea. 

Special Flight. Various flight patterns of birds may properly be 
considered as special flight, though they may be but variations of 
ordinary flying. Among these should be mentioned the helicopter- 
like action of a few birds. Chimney Swifts may flutter up a chimney 
in the morning or down at night. A Hummingbird may move ver- 
tically (Fig. 15-5). Several other birds, such as Larks and Grouse, 
have been observed to rise vertically in song or drumming flight for 
short distances (pages 283, 324, 329). Some birds can hover in the 
air to seize an insect, to observe, or for other purposes. In addition to 
Hummingbirds, hovering has been noted in the American Sparrow 
Falcon, American Marsh Harrier, American Rough-legged Hawk, 
Black-capped Chickadee, Golden-crowned Kinglet, Eastern Bluebird, 
Belted Kingfisher, Horned Lark, Lark Bunting, and many others. 


Reverse or backward flight seems possible in a few birds. Several 
birds in fights with their fellows have been noted to move backward 
in air. But the Hummingbirds use reverse flight as part of their regular 
daily habit. Hummingbirds withdraw their long bills from flowers by 
flying backward. Some Flycatchers (Tyrannidae), and possibly a 
few birds that can hover in the air, have been reported to reverse 


Feeding Flight. While birds as a group use their wings to trans- 
port themselves from one feeding spot to another, perhaps from limb 
to limb (as in the Warblers), perhaps 50 miles out to search for food 
(as in the ill-fated Passenger Pigeon), or perhaps far over the seas (as 
in oceanic birds) , many use their wings to place them directly in touch 
with their food, in many cases by sheer strength of flying. Swallows 
feed upon the wing, though they spend much of the day perched as 
do other birds. Chimney Swifts stay aloft for long periods (page 
191). The Nighthawk goes out to feed when the light is low, as at 
sundown and on dark days. Insect-eating birds that feed in flight 
tend to have wide bills that admit large insects and make a "near 
miss" of the target still effective. 

Gulls, Terns, and a host of other birds fly over the water until they 
locate suitable food, whereupon they seize it from the air or alight on 
the water to take it. In a sense, Vultures act somewhat similarly over 
land search from the air until they sight food, then land to feed. 
Some flights of sea birds in search of food exhibit the highest quality 
of airmanship. The Leach Petrel makes long excursions far out to sea 
where it skims along the surface barely above the waves. To take 
full advantage of the air currents immediately above the sea and espe- 
cially to hover helicopter-like, it has developed relatively large wing 
areas. Though of about the same weight as a House Sparrow, its 
wings are about the size of those of an American Robin or even 
slightly larger (Table 15-3). Its wing loading is about 0.2 of a pound 
per square foot, comparable to that of many bats, which range from 0.1 
to 0.3 (Poole, 1936). Even a Turkey Vulture has a loading of about 
1 pound per square foot. But the superior airfoil of the bird makes 
possible a greater efficiency with a heavier wing load than that devel- 
oped by bats. Birds fly faster than bats and a greater wing loading 
can be used when speeds are higher. 

Buzzard-Hawks fly, often by soaring, over the land much as do 
many waterbirds over the sea, searching for prey upon .which to 
pounce. But the Accpitrines usually get their prey by a sudden 
charge, giving up if the prey reaches a thicket, though they may 



pursue through open brush. The Falcons, however, catch their prey 
by "stooping" after "towering" or by overtaking the prey in open 
and continued pursuit. 

Display, Song, and Courtship Flight. As would be expected, 
ability to fly has led many birds to evolve flying maneuvers, often 
most spectacular aerial acrobatics, for display and song purposes (see 
Chapter 17). Display flights are more characteristic of prairie and 
tundra birds than of others, but they may occur among birds of tree- 
tops, marsh, and shore. Even birds of the brush and woodland may 
have display or song flights (see Fig. 17-8), as may Ducks and others 
of the water. Such flights often combine flight action and feather 
structures, but colored markings avoid parts subject to mechanical 
strain (Auber and Mason, 1955). Often it is difficult to distinguish 
display and courtship flight as such; probably all ranges of overlap 
wipe out any but the most apparent distinctions. Because both are 
given elsewhere, they will not be discussed fully here. Some species 
having song, display, or courtship flight arc listed below, together 
with their usual habitat. 


Vermilion Flycatcher 




Lazuli Bunting 

Song Sparrow 



Spotted Sandpiper 






Rough-legged Hawk 
Snowy Owl 

Lapland Longspur 
Snow Bunting 

Prairie Falcon 

Short-eared Owl 
Horned Lark 
Lark Bunting 

rileatea woodpecker 

Red-breasted Nuthatch 

Tree Duck 
Mottled Duck 
Blue-winged Teal 

American Marsh Harrier 

Long-billed Marsh Wren 

Red-winged Blackbird 
Swamp Sparrow 

American Sparrow Falcon 
Mourning Dove 
Scissor-tailed Flycatcher 


Environment and Flight Modifications. Many flight modifica- 
tions help make it possible for birds to radiate successfully into the 
many habitats available. Birds of the open develop highl^efficient 
flight, but those of specialized habitats may sacrifice aerial efficiency 
for more immediate advantages. The flight of gallinaceous birds is an 
inefficient type from the aeronautical standpoint, but it serves ad- 
mirably to get a heavy bird up and with celerity behind an obstruct- 
ing bush, limb, or tree. While the bird usually does not have to*fly 
far, it needs to make sudden departures on short notice. 

Birds requiring fast flight or great aerial agility have developed 
high-speed wings with narrow, swept-back leading edges, slight 
camber, and fairing of trailing edges (Savile, 1950). Ducks, Falcons, 
Plovers, Sandpipers, Swifts, Hummingbirds, and Swallows have inde- 
pendently evolved this type of wing (Fig. 15-5), an example of 
convergent evolution. 

As mentioned earlier, the longer tail of birds that hop from limb to 
limb, such as the Mockingbird, Chickadee, Chachalaca, and Cuckoo, 
help to support the body during short flights. They may also help 
to protect the feet from sudden jars by increased braking power for 
alighting (Fig. 15-7). The long tail of the Road-runner, however, 
may not be adaptive but a family character retained from the ancestral 
Cuckoo; it may help in twisting and turning, nevertheless, as when 
the bird pursues a lizard or engages in combat with a rattlesnake. 
Grouse of somewhat similar habitat likewise have long tails, which 
suggests that the long tail of ground birds may have some functional 

Large birds like the Canada Goose and White Pelican sometimes 
spend the night on rivers or lakes from which they must climb over 
a mountain or canyon wall to feeding grounds. The flocks often 
circle to gain altitude and sometimes move along the front of a canyon 
side until they can turn up a gulch or side canyon. Aerial evolutions 
of such flocks are common sights in many parts of the western states. 

Birds that migrate long distances tend to have more pronounced 
emargination of the outer primaries, which seems correlated with their 
greater need for flight power. Birds of the open have straight, direct 
flight, but those of brush or forest may have a more rambljpg flight. 
Nomadic birds and those that feed in large flocks often develop a 
"nervous" type of feeding and flight habits. Flocks of Snow Buntings, 
Bohemian Waxwings, Crackles, and Bush-tits may suddenly take 
wing without apparent cause, though the habit suggests a functional 
character (Miller, 1922). 

Owls show several modifications of flight related to their habitat. 
They fly after dark, though Short-eared and Snowy Owls, for ex- 





ample, may feed in the daytime, especially in high latitudes where the 
Snowy Owl may see no darkness for weeks. The soft plumage of 
the feathers serves to deaden sounds made by the passage of the body 
through the air stream; primary feathers are especially fringed to pre- 
vent or to muffle sounds. Because the Owl must usually hunt at close 
range on account of darkness, it has developed a characteristic, some- 
what butterfly-like flight gait. 

Flight Gaits. Just as we can sometimes identify a man by the way 
he walks, so also can we identify many birds by the way they fly. 
The undulating flight of Woodpeckers, often accompanied by wing 
noises, is characteristic; Nuthatches have similar flight. The foraging 
Brown Creeper seldom moves farther than from the top of one tree 
trunk to the base of another. The Eastern Goldfinch, American 
Robin, Red-winged Blackbird, and many others fly by a series of 
quick wing flaps, followed by a short glide. The Eastern Goldfinch 
alternately rises and falls as it makes a few rapid wing beats followed 
by a pause. The Red-winged Blackbird rises and falls much less, 
while Crackles scull across the sky. 

A comparison of the flight gaits among several common Swallows 
of eastern America shows in an interesting way their distinctive 
manners of flight (Fig. 15-8). The Tree Swallows sail in rather small 
circles, 20 to 100 feet or more in diameter. Their lower speeds result 




Fig. 15-8. Gliding attitudes of Swallows: (a) Tree, (b) Bank, (c) 
Cliff, (d) Barn Swallow, and (e) Purple Martin. (After Charles N. Blake, 
"The Flight of Swallows," Auk, 65(1948):54-62.) 


in apparent unsteadiness and numerous trimming wing flappings. The 
outstanding characteristic of the Bank Swallow is its fluttery, almost 
butterfly-like flight; it glides but little or for very short intervals 
during its irregular flight. The flight of the Rough-winged Swallow 
resembles that of the Barn and Cliff Swallows more than that of the 
Bank. The Barn Swallow courses in long runs, often near the ground; 
it may double back upon its flight path. It glides little if at all, and 
seems to have two styles of flight: coursing and quick flapping. The 
flight gait of the Cliff Swallow (page 278) is perhaps best described 
as intermediate between that of the Tree and Barn Swallows. It uses 
short glides with downward slanted wings. At intervals in its flight, 
it climbs steeply on rapidly beating wings, only to dive or flutter 
downward. The flight of the Purple Martin resembles that of the 
Tree Swallow; it sails in circles with an alternation of quick flapping 
and gliding and uses the tail more frequently than other Swallows. 
Similar variations in flight gaits, including the variable ways the head, 
wing, tail, and feet are held, characterize birds of close relationship 
and sometimes those of similar environment. Some bird observers are 
especially adept at using flight gaits for identifying Ducks, even at 
the very limits of vision. 

Erickson ( 1955) describes a flight behavior of the Procellariiformes 
called "Water Walking." In the most common form, the wings act 
as gliders while the bird runs over the water. The feet may also 
"run" sometimes in flight, presumably as a vestigial behavior. 

Flight Speed. The flight speed has long been commented upon, 
and some indications of speeds have been accumulated (e.g., Cooke, 
1937; Cottam, Williams, and Sooter, 1942; and Meinertzhagen, 
1955). The air speed of a bird is the speed with which it flies in 
relation to the air, ground speed with relation to the earth. The wind 
and many other things influence the observed ground speed, so that 
speed stated for some birds may be incorrect in revealing the actual 
facts (Allen, 1939). The bird flying in air moves as a part of the air 
stream. Hence, a bird flying 30 miles an hour with a 40-mile wind 
has an air speed of 30 and a forward ground speed of 70 miles per 
hour. If it is flying against the wind, however, its air speed would 
still remain the same but the ground speed would be 1 miles an hour 
backward. If the bird were flying without a "ground" reference (as 
in darkness), the flying effort would be the same in both cases. 

Some average flying speeds for several groups have been reported 
substantially as follows (ground speed): 

Small Passerines 20-37 Starlings 38-49 

Corvidae 25-50 Geese 42-55 

Shorebirds 35-50 Falcons 45-65 



A bird flying at cruising speed may not approach at all near its 
maximum speed, t In general, the pressed speed is about twice the 
cruising speed. The slowest powered flight accurately measured 
seems to be that of 5 miles an hour for the Woodcock. A few ex- 
amples, presumably correct, of easy or cruising speeds and those 
under urgency appear in Table 15*7. The maximum reliable speed 
for any bird is 94.3 miles per hour for a Homing Pigeon (Meinertz- 
hagen, 1955). 

Table 15-7 
Some Reported Ground Speeds. 


Cruising Pressed 

Great Blue Heron 18 36 

Black-crowned Night Heron 18 35 

Whistling Swan 30 50-55 

Canada Goose 44 60 

Mallard Duck 40 60 

Cinnamon Teal 32 59 

Red-head Duck 31 50-55 

Peregrine Falcon 37 75 

European Partridge 25 41 

Bob-white 28 49 

Red-shafted Flicker 25 44 

Black-billed Magpie 19 35 

House Sparrow 24 33 


*AYMAR, GORDON C., Bird Flight. New York: Dodd, Mead & Co., Inc., 1935. 
HANKIN, E. H., Animal Flight. London-, IHffe & Sons, 1913. 
JONES, BRADLEY, Elements of Practical Aerodynamics. New York: John Wiley & 

Sons, Inc., 1950. 
SHERWOOD, A. WILEY, Aerodynamics. New York: McGraw-Hill Book Co., Inc., 

SLIJPER, E. J., De Fliegkunst in bet Dierenrijk. Leiden, The Netherlands: E. J. Brill, 

*STORER, JOHN H., The Flight of Birds. Bloomfield Hills, Mich.: Cranbrook Institute 

of Science, 1948. 

t Reports of high speeds of flight are open to question. The Peregrine Falcon has 
been reported at 175-180 miles an hour (Cooke, 1937) and a Swift of India at 200 miles 
an hour (British Birds, 16:31, 1922). The observer reporting the latter based his calcu- 
lation on the time it took birds to fly from his position to a ridge two miles away, be- 
hind which he assumed that they disappeared. Tests have shown that even with 
binoculars birds the size of these Swifts cannot be traced at half the distance reported 
(page 465). 


Bird Migration 

Although writings, modern and ancient, may speak of the "mys- 
teries of migration," anything really mysterious about migration de- 
pends largely on our ignorance. The "mysteries" become fewer year 
by year. When more facts lie revealed before us, we shall doubtless 
be able to interpret the workings of bird migration in accordance 
with known principles and processes, such as of psychology, physi- 
ology, geography, meteorology, and physics. Bird migration occurs 
on so large a scale in high and middle latitudes as to become apparent 
to all who have contact with nature. The movement over great dis- 
tances has long challenged the imagination of men, for surely bird 
migration is one of the most remarkable happenings of all animal life. 
It is not surprising, therefore, that many explanations (both ancient 
and sometimes even more recent) of this, as of other biological 
phenomena, have at various times and places been clouded by super- 
stition, misconception, and fantasy. Though bird migration has 
claimed most attention, birds are not alone in migrating. The phe- 
nomenon of migration will be found throughout the animal world. 


Early Accounts of Migration.Qlistorical accounts tell of early 
people who remarked upon bird migration and sometimes used it as an 
indication of the coming of spring or fall. Writings of modern 
travelers have reported that among primitive peoples of today bird 
migration marks the advent of a new season, and probably the same 
may be assumed for prehistoric man. % The legends and literature of 
ancients contain references to bird migration. Thus we find Homer, 
the Old Testament, the Kalevala, and the Sacred Books of the East 
commenting upon the passage of birds. No doubt unwritten folklore 



also mentioned it. Aristotle (B.C. 384-322) wrote of migration and 
surely must have assembled much of his knowledge from both earlier 
and contemporary scholars and nonscholars alike. Pliny (23-79 A.D.) 
included migration comments in his Historia Naturalis, much from 
Aristotle but some his own. It is said that he had available a bio- 
logical library of 2,000 volumes, so that we may assume he built upon 
the writings of others along with information then currently available. 
Little increase in knowledge seems to have occurred during the* period 
of the Dark Ages. Frederick II (1194-1250), however, mentions 
migration in his famed De Arte Venandi cum Avibus, some of his 
comments being particularly keen. 

The industrial revolution brought an increase in avocational natural 
history as time for leisurely scientific pursuits became available with 
improved economic and political conditions; our knowledge of bird 
life, including migration, increased in consequence. By 1703, "a 
gentleman of piety and learning" had the courage to propound the 
remarkable fantasy that birds migrated to the moon by aiming in the 
right direction and flying a couple of months until the moon returned 
to the target position. Probably the gentleman would have been 
treated in a most ungentlemanly manner in the authoritarian days of 
a few generations earlier. Yet a hundred years later, knowledge of 
the pattern of migration was essentially modern, though many refine- 
ments, extensions, and improvements have been added since. 

Early Ideas of Migration. Just how some of the early ideas arose 
to explain the disappearance of birds in the autumn and their reap- 
pearance in the spring seems difficult to imagine. Most dealt with the 
mechanics of migration, though some may have been concerned with 
its origin.- No doubt many ideas have not survived to the present time 
because they were not written or because they were of local distribu- 
tion. The idea that birds flew to the moon in the fall and back again 
in the spring, though credited to the "gentleman of piety and learn- 
ing," probably existed in the unwritten lore of the time. 

| Origin of the idea of hibernation [among birds as an explanation 
for their disappearance in winter seems particularly difficult for us to 
imagines It has been suggested that the idea arose from the known 
fact of hibernation in many animal groups combined with the con- 
gregating of Swallows in reed marshes in the fall. Their early morning 
departure being unnoticed, it might be easy to imagine their settling 
into the water and mud to hibernate like turtles and frogs. Stories of 
torpid Swallows being dragged from the mud or caught in fishermen's 
nets can be credited to tellers of tall tales. 

CThe absence of many summer birds in winter and winter birds in 
summer gave rise to the seemingly reasonable assumption that one 


transformed into the other, the idea of ftj&^fl&M^ The rise of 
transmutation seems not at all surprising when we recall that things 
like supernatural power, fairies, miracles, and magic were part of the 
beliefs of folk people in many lands, widespread in the past and to 
some extent still. Because a fairy, according to myth, could change 
itself into a human being, it seems easy to imagine so simple a magical 
thing as the transformation of one bird into another. The known 
molt of some birds into different looking plumages may have helped 
propagate this belief in transmutation.j Many an early writer dis- 
coursed learnedly upon the matter. 

The apparent weak flight powers of some birds, especially small 
ones, influenced also the thinking of the time. That larger birds might 
travel far and over bodies of water, such as the Mediterranean Sea, 
was held to be possible; the passage of Cranes, Storks, and Geese to 
the Nile was known. The belief. that. small birds "hitch-hiked" on 
large-ones- ACuiint<e(^ of seerniitgly 

weak flight. As with hibernation ana other natural* evcfits,'" u eye wit- 
nesses" were present and able to validate the freighting of small birds 
by large ones. Eye-witness accounts of impossible happenings and 
imaginative interpretations of possible ones are not a monopoly of 
the ancients, as fiction and newspaper accounts of arrivals at missions 
and of other events of natural history sometimes testify today. Seem- 
ingly no less fanciful than the accounts of the ancients are "fossil 
flight plans" postulating that migration of birds originated in conti- 
nental drift. 

Origin of Bird Migration. The origin of migration belongs to the 
unknown of the past, and what we may learn of it is limited to deduc- 
tion from our knowledge of the life of the bird and of other animals. 
.No postulate of the origin of migration can do more as yet than offer 
us a suggestion.] In general, two theories have won some acceptance, 
largely perhaps because they were novel enough to seem somewhat 
satisfactory. f One theory holds that birdsJoJthejr northern homes 
were forced southward by advancing glacial or other unfavorable 
conditions. With retreat of the ice, birds whose ancestral homes were 
in the North came back but had to leave each fall. In a sense then, 
each species annually follows the ancestral north and south movement. 
The converse theory holds a southern ancestral home and that species 
pushed northward only to be driven back again, a condition now a 
racial habitjt 

Some very definite and real facts make both theories unsound ones. 
Birds of North American origin, the Warblers (Parulidae) and Vireos 
(Vireonidae), for example, migrate into South America, which is 
wholly counter to the theory of return-to-the-southern-ancestral- 


home (at least from the family view). The several common Gros- 
beaks and Buntings of the subfamily Richmondeninae, the Humming- 
birds, the Tyrant Flycatchers, and the Tanangers appear to have arisen 
in South America, and their wintering in South America may fit the 
southern-ancestral-home theory but runs counter to the northern- 
ancestral-home one. ( The fact that some members of each group do 
not migrate, or migrate little, runs counter to both.l It hardly seems 
within the bounds of proper logic to sort out those facts in agreerhent 
with each as proof for the validity of the respective theory. The 
fundamental error of both seems to be considering bird migration as 
peculiar only to birds of the less temperate parts of the Northern 

\The Oropendolas of tropical Panama, for example, leave after the 
breeding season and return again next year. The White-winged Doves 
south of the Tropic of Cancer in Mexico move out after the nesting 
season, just as do those farther north. The Yellow-green Vireo of 
tropical Central America migrates like its northern relatives. The 
Gray Kingbird of the West Indies even reaches the Amazon Valley 
in migration. These few examples serve to show that migration does 
occur in tropical regions; it may prove far more regular there than 
has been commonly assumed )(Fig. 16-1). 

( In addition to many land birds, pelagic birds of tropical and sub- 
tropical waters also show definite migrations. The Tropic-Bird, Alba- 
trosses, Shearwaters, and others may wander far over the tropical 
seas yet return to small islands to nest. The Wide-awake of Ascen- 
sion Island has a habit, remarkable to us, of returning at nine- to ten- 
month intervals to breed four times in about 3 years (page 358). 

Biological Origin of Migration. ' The fact that seasonal move- 
ments occur throughout the entire animal kingdom, both invertebrate 
and vertebrate, suggests the very plausible working hypothesis that 
such movements are an inherent part of animal mafce-jip\)All grada- 
tions occur between the few inches of movement of a tick seeking 
winter quarters in the forest floor and the great intercontinental mi- 
grations of birds. win many cases, the movement of an invertebrate 
seems just as great a tax on its locomotor powers as the interconti- 
nental journey is to a bird. / A snake that slithers 5 miles down a 
canyon bottom may have maoe a journey comparable to 200 miles or 
more for a Dove. The toad that hops 2 miles to a pond for breeding 
has performed a rather long journey.^On theoretical grounds, it seems 
entirely logical that when amphibians arose from fish, they carried 
with them a hereditary tendency for seasonal movement that they 
passed on to reptiles which in turn passed it on to birds and mammals. 
Even though some fish and mammals migrate (e.g., salmon, eel, bat, 




caribou, seal), birds with their great mobility have brought migration 
to a state of geographic perfection previously unknown/ It is pos- 
sible that bird migration arose by the application of greater distances 
to an existing biological inheritance of periodic movement during 
unfavorable seasons.^ 


Internal lnfluences(Controls .of birdjroigiaUQru.w.hiJe more diffi- 
cult to recognize than tKbse of common activities, seem to center in 
the nervous and endocrine systems, the two coordinating mechanisms 



Feb. Mar. 

Fig. 1 6 2. Migratory birds lay on fat before migrating, often while 
resident birds are losing it during the stress of the breeding season, as 
shown by curves of body weight. (After Albert Wolf son, "The Role of 
the Pituitary, Fat Deposition, and Body Weight in Bird Migration, 11 
Condor, 47(194J):9f-127.) 

in the body. The over-all control of the endocrine glands rests in the 
gitm^ty. Experiments have shown that increase of light through 
affiffioauy increasing the length of day causes the pituitary to secrete 
when the glands are increasing in size, as would happen in spring. 
With the growth of gonadal tissues often goes increased deposition 
of 'fat, like fuel being loaded for a journey, sometimes 10 per cent of 
the body weight in 7 to 10 days (Fig. 16-2) (Wolfson, 1945; Odum, 
1949). Probably other functioning parts of the body are likewise 
brought into migratory adjustmen^ (Seibert, 1949). 


When the physiological condition of the bird reaches the right 
stage for the bird to cope with the strains of migration, release of 
the appropriate behavior patterns seems to occur (see Fig. 1 3 l).(Trte 
migratory tendency and the functioning of the body are inherent 
behavior patterns set in motion by external, annual stimuli, of which 
changes in day length are believed to be the most impqrtanpl Not all 
bir ds respond "aKKe"; resident birds may not respond as do migratory 
ones of the same species^ Sudden changes seem unable to cause re- 
sponse, which seems adjusted to progressive, slow changes/The stimu- 
lation of migration evidently may involve the nervous system as shown 
in penned birds. Confined migratory birds (and sometimes resident 
ones also) show signs of tension during the migration season; they are 
particularly restless during the hours of peak migration) (the Zugim- 
ruhe of Palmgren, 1944). 

External Influences. (Birds in high latitudes experience a marked 
change of day length \photoperiod) in midsummer when rather 
abrupt fall migration behavior becomes clearly evident. The shorten- 
ing of the day, however, is not so noticeable in lower latitudes? 
The many northern birds wintering south of the Equator raise a 
definite problem to the assumption that increased day length initiates 
spring migration. For them, the length of the day decreases instead 
of increasing as ft does north of the Equator. But we arc unable as 
yet to distinguish between possible stimulation by relative day length 
and by a c ff gffgtogflL JSZJgPfi *h The stimulation of the pituitary by 
day TeRg^^ or decreasing light, may be a 

cumulative one regulated by the total length of the days.^Experi- 
ments with captive birds tend to confirm the hypothesis that total 
day length, within the required limits and following a refractory 
period, stimulates the pituitary (Wolf son, 1952), but the alternation 
of light and dark seems more effective than continuous lighty 

/'Temperature and humidity no doubt influence migratioiy (though 
trie bird is so highly insulated as to be considered rather independent 
of temperature change) . Birds flying northward may be shifted about 
by wind and storm, so that actual arrival at any spot may be from 
any compass direction, though the migration itself may be from the 
south.) In like manner, birds may depart in almost any direction from 
the local standpoint, though the over-all direction is north or south. 
It may be possible, though we know little about it, that birds mi- 
grating in spring can recognize warm^wet air masses flowing north- 
ward or coldr drvones rolling out of the North. The same may 
occur m^ thejall. (The arrival of unusual numbers of birds and even 
their appearance off course may correspond with air mass movements 
as well as with barometric pressures.^ 


Atlantic Golden Plover 
breeding range 

Pacific Golden Plover 
breeding range 

[ 1 Winter range 

Fig. 16* 3. Adult Golden Plovers migrate, presumably nonstop^ across 
the western Atlantic to South America in the jail and northward through 
the Mississippi Valley in the Spring. Young birds use the Mississippi route 
during their first jail. The Pacific Golden Plover and some other species of 
Alaska and Siberia apparently make a nonstop flight across the ocean to 
the Hawaiian Islands, Marquesa Islands, and Low Archipelago. (From 
Frederick C. Lincoln, Migration of Birds, 17. S. Department of the In- 
terior, Fish and Wildltfe Service, Circular No. 16(1950), p. 54.) 


(in addition to air masses, the ^anetar^'wind system also seems to 
jxert a control ov&c j^ig^atioh. jS'he Golden Plover st 

exert ^control ov&c jftigEatiofl. jQ."he Golden Plover starting from 
the Arctic, for example, appears to drift eastward in autumn with 
the prevailing westerlies shifted north with the sun, but they are also 
being turned southward by deflectioh, which helps to bring them into 
the New England and Maritime coasts. /It nlust be recognized also 
that the birds probably follow landmarks suitable to their instinct 
pattern.* Once taking off south across the western Atlantic, the 
Golden 'Plovers should come under the Northeast Trades that bear 
them into South America, rather than out into the South Atlantic 
(Fig. 16-3). In addition, they move southwest by deflection. Once 
across the Equator, the Southeast Trades also tend to drift them west- 
ward so that their appearance in Patagonia seems assured. The 
journey north again drifts them westward, toward Central America 
rather than the Atlantic Coast. Their appearance on the plains and 
in the Mississippi Valley thus seems reasonable. JNo doubt landmarks 
are used in maintaining a course within the general northward im- 
pulse, though local and regional movement may be influenced still by 
wind, air masses, and storm.! But how Alaskan birds (several species 
besides the Golden Plover y hit Pacific islands is not so readily ex- 
plained as how those of eastern America hit South America (Fig. 


Means of M(gration.^Birds migrate by exactly the same means 
that they use for ordinary everyday passage walking, swimming, 
flying. Many gallinaceous birds, if they migrate at all, migrate on 
foot.) In eastern North America, the Wild Turkey in the early days 
migrated on foot, perhaps a hundred miles or more, flying only when 
it crossed rivers or left its night roostYThe Blue Grouse of the 
Western Mountains migrates up the mountain slopes on foot in fall 
and down in the spring, mostly on foot) (Fig. 16-4; see also page 301), 
a journey that may reach 15 or 20 miles (Wing, 1947). But the 
Prairie Chicken and Sharp-tailed Grouse fly in migration, ^Flightless 
land birds, like the Ostrich and Rhea, perform on foot whatever sea- 
sonal movements they may haveJ) Flightless sea birds migrate by 
swimmingA The Great Auk migrated southward along both sides of 
the Atlantic and at times reached the Middle Atlantic States and 
France. The Penguins of the Antarctic migrate away from that con- 
tinent and back again by swimming. 

0n a few known cases, migrating birds moved by long and rapid 
daily flights^) A Blue-winged Teal (young male) was recovered at 



Maracaibo, Venezuela, exactly 1 month after its banding 3,800 miles 
away in the Athabasca River delta (Lincoln, 1950) . A Lesser Yellow- 
legs banded on Cape Cod, Massachusetts, August 28, 1935, was killed 
6 days later on Martinique, West Indies, 1,900 miles away. Its aver- 
age speed of 316 miles a day establishes a record. Migrating bands of 
Hawks, moving along the Allegheny ridges may go long distances 
without stopping. Flights of Herons and Geese have been followed 


Both areas used by males during July-August , 
and by all birds September -March 

Used chiefly by adult Grouse 
during May- June 

Used chiefly by females with 
broods in July-August 

Infrequently used by 

Blue Grouse. 

(Ruffed Grouse habitat) 


01 23 


Fig. 16*4. The sivnnner and 'winter ranges of the Blue Grouse in Idaho 
show seasonal preferences. (After William H. Marshall, "Cover Prefer- 
ences, Seasonal Movements, and Food Habits of Richardson's and Ruffed 
Grouse in Southern Idaho," Wilson Bulletin, 58(1946):42-52.) 

for many miles across country. A banded Turnstone released at 
11 A.M. at Helgoland was shot 25 hours later on the North Coast of 
France, 5 1 miles away, the fastest flight yet known for a single day. 
Some flights of homing birds, it might be added, have reached greater 
daily speeds: 715 miles in fewer than 24 hours for a Herring Gull 
and 1,000 miles for a racing pigeon (Griffin, 1944, 1952). But most 
migrations of small birds seem less hurried. The American Robin 
advances northward hardly as fast as the march of spring. Canada 
Geese move at about the speed of spring and follow rather faithfully 
the 35 F. isotherm. The Black and White Warbler averages about 



Breeding range 
Winter range 

Fig. 16-5. Breeding and winter ranges of the Black and White Warbler. 
Isochronal lines show the northward movement. (From Frederick C. 
Lincoln, Migration of Birds, 7. S. Department of Agriculture, Fish and 
Wildlife Service, Circular No. 16(1950), p. 13.) 

20 miles a day in crossing the United States (Fig. 16-5); the Black- 
poll Warbler averages 30 miles until near its breeding range, at which 
time it may speed up to 200 miles a day (Lincoln, 1950). The Gray- 
cheeked Thrush may average 130 miles a day during its 4,000-mile 

A bird may take 6 weeks to cross the United States and then use but 
10 days to pass on to Alaska. (Toward the end of migration, birds 


hurry through in mid-latitudes much more rapidly than earlier in the 
season. Acceleration of speed as migration draws to a close or as the 
bird approaches the breeding grounds is a regular characteristic. The 
mechanism controlling this and the variations involved have been 
little studied; consequently, we know little of this important accelera- 
tion process^ 

(in general, many birds migrate at an average rate of but 30 or so 
miles a day. Birds may pause in migration^nd banders oftenTincT 
' flRff'Bff 9s Repeating for 2 or 3 days or even a week before a new 
group replaces them. Fall migrations for small birds may be rather 
deliberate and in easy stages compared to spring. But some birds, such 
as some Waterfowl, may remain in the North until late and then rush 
southward. Yet some migrants may move southward very rapidly in 
early fall. 

Kinds of Migration. Migration from areas of severe winters to 
lands of mild winters has claimed most attention. Yet it appears likely 
that(pnly some 20 per cent of the world's birds so migrate, with per- 
haps 60 per cent more wandering at least locally in the nonnesting 
season. Not more than 20 per cent should be accounted wholly 
sedentary and fixed the year around. No classification of birds ac- 
cording to migratory habits fits all cases or even the same bird every- 
where. A common practice, however, lists birds under such terms as 
transients, permanent residents, mimner residents, winter residents, and 

Tri some birds, as in the Black-capped Chickadee, Song Sparrow, 
and Cowbird of North America or the Starling, Rook, and Skylark 
of Europe, some individuals or some populations may be essentially 
resident while others may migrate. It appears that all gradations occur 
between long geographic migrations and confinement to a fixed ter- 
ritory the year around. Some birds may in various areas and at vari- 
ous times show several different movements, in some cases staying 
over one winter and migrating another year. 

Local shifts of range occur in many species. To feeding stations 
in winter come wandering Black-capped Chickadees that banding 
shows remain in the region throughout the year. Yet careful banding 
during the winter sometimes reveals two or three complete or nearly 
complete replacements by influx of new birds and departure of fa- 
miliar ones. It has been presumed that the more sedentary birds are 
older ones. In the American West, Prairie Falcons may move out 
into the open fields in winter, though the rest of the year may be 
spent near bluffs and other nesting sites. Birds of the forest abandon 
the more exposed (especially more windswept) areas and pass to the 
thicker timber in winter. A particularly cold period, as occurs with 


passage of a cold air mass, may bring about a shift to more protected 
cover. There may be daily or seasonal shifts to warm layers of air 
in mountain regions. Cardinals sometimes move in winter to brushy 
river bottoms and Song Sparrows to cattail marshes. Coveys of Bob- 
whites gather in fall and wander rather systematically over a circum- 
scribed winter range. 

In mountain regions, many birds shift in winter from higher to 
lower altitudes. The birds of higher reaches move to lower altitudes 
and foothills and sometimes far out on the adjoining countryside. In 
the Rocky Mountains and Cascades, Juncos, Grosbeaks, Rosy Finches, 
and others reach the lower altitudes in large flocks in fall and winter. 
The Rosy Finches may drop 10,000 feet or more in altitude yet move 
scarcely a hundred miles. Parallel shifts in the Tropics have been 
reported; the Emerald Toucanet breeds near the tops of the moun- 
tains from November or December to May or June and passes the 
nonbreeding season in the lower altitudes (Fig. 16-1). 

The Clark Nutcracker of western forests breeds early in summer 
and moves upward with the advance of summer but returns before 
winter. The Blue Grouse, however, reverses the usual practice and 
descends to breed in the low altitudes and moves back to winter again 
in the high altitudes (Fig. 16-4). The birds trek on foot, during the 
downward journey occasionally flying across open slopes. 
\Sporadic migration seems to occur in some species. Pallas Sand 
Grouse often "irrupts" out of the central Asia dryjands in a north- 
westerly direction to reach Europe and even the British Isles. Among 
the invasions known were large ones in 1863, 1888, and 1908. Under 
sporadic migration may perhaps be grouped such events as the inva- 
sion of North America by Lapwings from Europe, as well as the 
European Widgeon (though the latter may breed in small numbers 
somewhere in the eastern, New World Arctic). The Thick-billed 
Parrot invaded southern Arizona from Mexico in July, 1927. 

/Accidental occurrence may be of the same nature as sporadic mi- 
gration, but most likely it indicates birds blown off their courses, 
those that may have missed or overshot the mark, and some that are 
probably confused wanderers.) 

/A special kind of migration in the Shelduck of Great Britain has 
been termed molt-migration (Coombes, 1950). During July, even 
before many young are flying, almost all the adults depart from the 
western side of the island to the eastern side by definite routes, show- 
ing great reluctance to fly over land. There they pass through the 
summer molt, during which time they become flightless like other 
Ducks and Geese. The birds drift back again slowly over a period 
of six months?) 


(jStill another movement that must be considered when dealing with 
migration goes under the name of nomadism. True nomadism in- 
volves absence of a fixed home and breeding wherever and whenever 
conditions are suitable) This condition seems to be met in part by 
Crossbills, especially the Red Crossbill, in eastern North America and 
parts of Europe and perhaps of Asiai}The Crossbills wander over the 

Fig. 16' 6. Recoveries of Bald Eagles banded in Florida, mostly as 
nestlings, show a northward movement after the nesting season. (After 
Charles L. Broley, "Migration and Nesting of Florida Bald Eagles" 
Wilson Bulletin 59(1941):!.) 

forest and nest in areas having a good cone year. They have even 
nested away from the conifer country, perhaps because of cone fail- 
ure. Nesting has been reported for every month of the year, though 
mot nesting still occurs during early spring. 

(jhe postseason wandering of young birds, especially the north- 
ward movement of young Herons, Egrets, Gulls, Bald Eagles, Mourn- 
ing Doves, and others, has been termed vagrant migration^Fig. 16-6). 


How widespread it may be is not known, but it may very well occur, 
at least occasionally, in most families of larger birds and perhaps 
others also. Banding returns indicate that it occurs more often than 
previously thought to be the case. 

Flight Years. To speak correctly, we should probably refer flight 
years to sporadic migration. The Crossbill has already been discussed 
as nomadic. Whether called nomadism or sporadic migration, flight 
years are so well known in a number of northern birds as to demand 
special recognition. No doubt all gradations can be found between 
migration and the invasion so striking as to be called flight years. 

|866 ( 

1022 1042 


( ( l i i i i i i 1 1 ' 

n i i 4 ,!, ...- 1 r - 

'" + IvJFJT^H 

i ' 

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i J i 1.1.1 i i i i ~~ 

Fig. 16-7. A 4-year time chart of the Pine Grosbeak flight years in 
the Great Lakes area shows a reversing cycle of 3.94 years. The rise of 
one year in the sixteen intervals between the arrows indicates a "false 
length" of one-sixteenth year shorter than the time chart intervals. The 
reversal points are marked by stars. Two cycles, one 4.222 years long 
and another reversing cycle of 3.69 years, are associated with this behavior. 
A reversing cycle is a wave behavior known also as a "beat." (From 
Leonard W. Wing, "Global Pattern of 4.222-Year Cycles in Tempera- 
ture," Journal of Cycle Research, 3 (19 5 4) :5 5-83.) 

Flight years of the Snowy Owl in eastern North America occur at 
about four-year intervals in a well-marked manifest cycle (Gross, 
1947). Those of the Northern Shrike are similar. Snowy Owl flights 
occurred in 1833, 1837, 1839, 1846, 1853, 1862, 1866, 1876, 1882, 
1886, 1889, 1892, 1896, 1901, 1905, 1909, 1912, 1917, 1921, 1926, 
1930, 1934, 1937, 1941, 1945, and 1949. Among other birds hav- 
ing similar flight years are Goshawk, Bohemian Waxwing, Evening 
Grosbeak, Pine Grosbeak, Red Crossbill, White-winged Crossbill, 
and Red-breasted Nuthatch in North America. Similar northern 
birds have flight years in the Old World, often the same or related 
species in the two continents. Flight years may also involve desert 
or dry land birds (though perhaps not in America). The Sand 
Grouse has long been known for this behavior. 

The flight years of the Pine Grosbeak in the Lake States (Fig. 
16-7) averages about 3.94 years apart as the result of two or more 
cycles (Wing, 1954) (which in turn is compounded of two or more 
other cycles). One cycle measures 4.222 years long and another 


3.767 years. In some species, the flight years may occur rather 
markedly every other year and show evidence of two-year cycles or 
alternations. Alternations were reported in the nesting of the Pas- 
senger Pigeons and were related to setting of the acorns the year be- 
fore (Schorger, 1937). (Some alternations in the plant world may be 
related to carbohydrate exhaustion, as in fruit trees, though we know 
little of it in the wild. Some alternation of breeding success of anynals 
has been termed "inversity.") 

Altitude of Migration.(^Birds migrate at low altitude and few mi- 
grate at distances more than 3,000 feet above the ground itself in 
fact, most migration passes within a few hundred feet of the surface, 
be it water, trees, or ground. Birds passing over the open sea have 
been reported flying close to^the waves, which can also be noted in 
birds migrating along coasts, (it seems entirely likely that the bulk of 
migration occurs at levels under 300 feet or so above the surface. 
Observations of migrating birds, scanty though they are, indicate" 
that birds follow the ground level, rising over ridges and dropping 
down into valleys) An observer in Abyssinia reported Swallows 
clearing a 10,000-foot ridge and passing down into the valley below. 
Since the country beyond the valley also* was high, it must be pre- 
sumed that the birds rose again on the far side of the valley. Birds 
riding the air currents along the windward side of mountain ridges 
may be expected to pass at any altitude, though their distance above 
the slopes is low. In the same way, birds may cross a narrow valley 
to ridges on the far side without losing altitude. Ridges of the Cas- 
cades reaching nearly 8,000 feet have been cleared by Canada Geese 
without dropping into a valley bottom 2 or 3 miles wide and but 
1,200 feet in altitude. Migrating Cranes have been reported over the 
Himalayas at heights in excess of about 14,000 feet (perhaps as high 
as 20,000 feet); the height above the surface still probably did not 
exceed 5,000 or 6,000 feet. 

Diurnal and Nocturnal Migration/ Most birds in migration show 
definite tendencies to be either diurmt or nocturnal, but some may 
be both. In general, birds of strong and direct flight, those of the 
open, fhose of large size, and those that can feed on the wing tend 
to migrate chiefly in the daytime. Birds of weaker flight, those of the 
brush and woodlands, those of small size, and those that musj: search 
for small food items near the ground migrate chiefly at night.)Water- 
fowl and Shorebirds have long been known for their habit of migrat- 
ing at night, especially on clear nights, as well as during the day. A 
list of probable nocturnal and diurnal migrants compiled frtfm various 
reports is given below: 



Rails, Coot 




Flycatchers (short-tailed) 















Flycatchers (long-tailed) 









Warblers (New World) 




Grosbeaks (Neotropical) 

Sparrows (native) 




American Crow 

American Robin 






Blackbirds, Cowbirds 

Purple Finch 

Grosbeaks (Boreal) 


Goldfinch, Siskins 




Flocks of migrating Geese, Crows, and Blackbirds are common 
sights over favored routes in eastern North America. Nighthawks 
migrate in large circling flocks, feeding as they go, wending their 
way always toward the ultimate destination. Grackles pass in long 
streaming flocks. But no doubt mass migration reached its greatest 
heights anywhere in migrations of the Passenger Pigeon which formed 

10 II 12 I 


Fig. 1 6*8. Average hourly densities of birds observed by telescopes 
sighted on the full moon, based on Central Standard Time (90th Meridian 
Time). (After George H. Loiuery, Jr., A Quantitative Study of the Noc- 
turnal Migration of Birds, University of Kansas, Museum of Natural His- 
tory Publications, 3(1951)361-412.) 


great rivers of living birds, miles wide and miles long, hundreds of 
millions of birds streaming onward (page 256). 

I Regular night migrants move in scattered fronts across the night 
sky. Thousands of birds may pass over in the course of a single night 
to descend to earth by the coming day. The sudden appearance of 
birds on any one day in the spring reflects arrival during the previous 
night. The training of telescopes against the moon during the nights 
of migration reveals this parade across the night sky (Lowery,*1951). 
The flow varies with the hour, place, season, and power of migration. 
In general, the rate of nocturnal migration increases from dark until 
near midnight when it reaches its peak (Fig. 16*8). Studies in 
Europe show that the greatest diurnal migration activity occurs after 
dawn and again in late afternoon, somewhat appropriately similar 
in pattern to night migration (Thomson, 1953), as well as normal 
diurnal activity."} 

Advantages of Migration. It may be taken as axiomatic that a 
phenomenon of such magnitude as migration has corresponding ad- 
vantages to the participants. The travels of most high-latitude mi- 
grants clearly makes it possible for them to occupy range suitable in 
the summer even though it may be wholly unsuitable in winter. 
Biological energy resources being what they are, high latitudes have 
their highest energy potential in summer, whereas equatorial regions 
have uniform resources, save for marked wet-dry season differences. 
Hence, the nearer one approaches the Tropics, the more nearly uni- 
form becomes the yearly energy distribution and the less becomes the 
migratory tendency, though we must not overlook movements dur- 
ing the nonb reeding season in low latitudes. Migration enables birds 
to utilize the cold latitudes in summer, for example, though they must 
go elsewhere for winter. 

An alternative to migration is hibernation, a method used by many 
reptiles; "amphibians, and mammals. Authentic cases of torpid con- 
dition during inclement weather, however, have been reported for 
one or more Goatsuckers (Caprimulgidae). 

It seems entirely probable that many birds migrate farther in the 
fall than absolutely essential for their basic needs. Many Flycatchers 
pass on to the Tropics, though the Eastern Phoebe stays farther north. 
Some Shorebirds and others may scatter out along coasts from the 
United States to Argentina. However it may have arisen, this winter 
spread may act as a safety device to prevent local overabundance and 
to distribute the species and family pressure more widely. 

Disadvantages of Migration. sMigmojcy:, birds. faee many- dan- 
gers and there is always the possibility of encountering, catastrophe as 


they travel. Storms have brought catastrophe to birds and at times 
thousands, even millions, have died. A bird resident in familiar sur- 
roundings would surely seem to have an advantage over one in strange 

Many migratory birds fly into towering objects at night, and the 
loss at some lighthouses has been high. The erection of perches has 
helped to prevent loss by allowing birds to rest and by reducing 
hovering flight, laborious for birds; change of the light to red is said 
to reduce, even eliminate, bird destruction. Many birds have flown 
into the Washington Monument, towering 555 feet skyward, but the 
number declined with growth of the city and consequent improved 
city illumination. At times, birds have crashed against giant sky- 
scrapers in cities. 

(JJome birds may arrive in the North only to be caught by a late 
storm and suffer hardship or annihilation. Particularly unfavorable 
cold in the winter range may also cause great loss. In the same way, 
migrating birds may run into an unseasonable storm. Many cases 
are known where such a condition resulted in great mortality and 
brought about reduced breeding populations for a few birds in some 
parts of the breeding range later.,) Unusually severe winter storms 
sweeping into the southeastern states have killed many birds, and 
their loss meant a scarcity in some of the northeastern states for some 
years!) A "norther" across the southeastern parts of Texas and adja- 
cent Louisiana in February, 1951, for example, brought death to 
Mourning Doves, Mockingbirds, and many others. 

Exhaustion from weariness of flight probably does not occur ex- 
cept possibly when a bird crosses a large body of water, or when it 
flies under unusual circumstances that drain its energy before replen- 
ishment by feeding. The stored energy of the Ruby-throated Hum- 
mingbird has been assumed to give it a flight range of 835 miles (Pear- 
son, 1950). The popular notion that birds always return from distant 
lands exhausted by their efforts seems to be erroneous, yet we should 
expect some birds to die along the route. 

Migratory Navigation. Navigation by migrating birds has been 
confused with homing by nonmigrants (e.g., Homing Pigeons) and 
with birds carried away from their homes (such as nesting Gulls, 
'Ducks, and others). While probably related, the navigation of mi- 
grating birds, sometimes for the first time, over unknown land to un- 
known winter grounds, is a different thing from returning to a home 
from which only recently removed. Nocturnal migrants, like any 
airborne body, may drift with the flow of air, which in southern parts 
of America and Europe means an easterly drift with the prevailing 
westerlies. Like other moving bodies, they are influenced by the forces 


of deflection (Coriolis force), to the right in the Northern and to the 
left in the Southern Hemisphere. 

Many diurnal migrants clearly show the influence of terrain as they 
migrate up river valleys, along shores, mountain ridges, and from 
favored cover. Sometimes this causes a reverse migration locally. 
Birds migrating northward around the "thumb" of Michigan may 
swing southward along the western side until they reach Sand Point, 
over which they go before launching their northward flight across 
Saginaw Bay. The concentrating effect of points in lakes and bays 
have long been known. Keweenaw Point (Lake Superior), Sand 
Point (Lake Huron), Fish Point (Lake Huron), Dorr Peninsula 
(Lake Michigan), Cape Cod (Massachusetts), and Cape May (New 
Jersey) have been famed for years as favorable observation places. 
Points of vegetation in desert regions, groves in prairies, islands in 
bays, and even mountain peaks concentrate birds in migration. 

Nocturnal migrants, too, seem influenced by terrain, for even at 
night, shadows show the ground pattern. Birds fly less on cloudy 
nights than on clear ones; they may run into storms or fogs and there- 
upon become confused and lost, chiefly because they have lost con- 
tact with the ground. Sometimes birds flying over cities, especially 
in fog, become lost, probably for the reason of pattern change. Light 
below where it should be dark, and darkness above where it should be 
light, presumably cause loss by the birds of their plane of reference. 
With sufficient visibility, the bird seems no more confused than in 
flight at twilight. 

Theories of Bird Navigation. Many theories have been advanced 
to account for birds finding their way. One involves a "mythical 
sense of direction," but nothing of this sort is known (except in the 
memory itself) . Just as some people seem better able to keep direction 
than others, birds may be better than most other animals, (it has been 
suggested that young birds learn from the older members of the 
flock, but for some species serious objections can be raised to this 
view. Night migrants do not seem to travel in flocks as diurnal mi- 
grants do, and unless in compact flocks, they probably could not fol- 
low each other except by vocal -contact, vjn any event, the young of 
some birds migrate separately j^the young Cowbird, for example, 
would hardly be expected to migrate with its foster or biological 
parents. Most young Golden Plovers, in fall migrate to South Amer- 
ica through the American interior, while the adults cross the western 
Atlantic (Lincoln, 1950). Next year these young would be adults 
migrating over the ocean. 

Birds that fly long distances (even across water bodies to land 
beyond the limits of vision, setting course to objectives a hundred and 


even thousands of miles away) cannot be credited with ocular navi- 
gation. Birds close to the ground have a limited horizon; those swim- 
ming in water like the Penguins would see ahead perhaps but a few 
himdred rods. 

(jBirds may be able to identify air currents by their quality or direc- 
tional flow when ground reference is present. They have no known 
way by which to sense or to be influenced by magnetic lines of force. 
It has been suggested that radar rays may influence them, though 
these are unknown in nature. That birds detect the earth's deflec- 
tion (Coriolis force) has been suggested, though no known structure 
for such detection of deflection exists. Aerophysicists estimate that the 
bird would have to be sensitive to one part in a million of gravita- 
tional acceleration in order to navigate by such force. Air turbulence 
would mask any signal of such a low order. In any event, the action 
of Coriolis force is a deflective one acting upon all moving objects, 
inanimate and animate alike. The movement of air in the tympanic 
cavity of the ear has been proposed as one possible detection method 
(Beecher, 195 la). 

The direction of bird migration in the Northern Hemisphere is 
chiefly equatorward in the fall and poleward in the spring. Young 
turtles are reported to move toward light horizons in making their 
way to water. Bees have been found to use the angle of light to orient 
themselves (menotaxis). It is suggested that birds also may use a form 
of sun navigation (Kramer, 1952; Matthews, 1955). (The pecten., 
a structure in the eye, is suggested as the device involved.) 

The wintering of northern birds south of the Equator raises a 
difficult point in explaining bird migration. One attempt suggests a 
totality of daylight, rather than immediate increase. Perhaps for now 
we should still consider migration among birds of northern and mid- 
dle latitudes as a directional response^ forward and back with the re- 
spective seasons. The actual control mechanism itself is surely a most 
complex one, not necessarily operating alike for all birds or at all 
times. The directional controls of the cycle need not be the same 
throughout the season, even in the same bird. So far as finding their 
way is concerned, the geographic cues available (such as wind, land- 
marks, vegetation, topography, skylight, sun) seem sufficient to pro- 
vide all the guides necessary if the birds can detect such cues, 


Migration Pathways. The migration routes of birds form such 
an integral part of the migration pattern that it seems hardly possible 
to discuss them separately, so that the subject actually has been cov- 




Recovery poinl 
Migration record 

Fig. I6 9. The route indicated for the Arctic Tern is unique; no 
other species crosses so freely over the Atlantic between the Old and Neiv 
Worlds. The extreme summer and winter homes are llfiOO miles apart, 
so that some Terns probably fly at least 25,000 miles a year. The recovery 
points represent birds banded as nestlings in North America. (From 
Frederick C. Lincoln, Migration of Birds, 17. 5. Department of the In- 
terior, Fish and Wildlife Service, Circular No. 16, p. 39.) 


ered already. A few points need to be elaborated upon, and it may be 
profitable to mention some unusual examples. 

The thought of "flyways" developed when banding returns first 
began to confirm the idea that birds tended to follow regular direc- 
tions. But the idea of four great Waterfowl "flyways" proves to be 
a better administrative convenience than a biological actuality. Hie 
four "flyways" envisioned as realities were the Pacific, Central, Missis- 
sippi, and Atlantic. Their prime use is convenience in dividing up 
the continent for Waterfowl regulations. 

Migration pathways and flyways can be seen on every hand dur- 
ing the passage of migrants at the height of the season;^ Birds will 
work through the woods in a flock and follow out along fence rows 
or concentrate in points of brush before striking out across the open. 
Coastal margins, points of timber, and land reaching out into marshes 
and bays arc well-used pathways. Coasts themselves form important 
flyways for birds. Water birds often follow rivers for long distances, 
even faithfully following river bends though it would be easier to 
cut across them. But some may cross mountain ranges to shorten dis- 
tance?^ The Ross Goose migrates down the eastern front of the 
Rockies from its restricted breeding grounds in the Perry River dis- 
trict before crossing to its restricted winter range in the interior valley 
of California. 

The round-trip flight of the Arctic Tern from its North American 
breeding grounds to its winter KomcTin the Antarctic and Sub-Ant- 
arctic has long interested people (Fig. 16-9). The birds of north- 
western North America migrate down the Pacific coast of North and 
South America. Those of the eastern half of the Continent cross the 
North Atlantic and pass down the European and African coasts. 
Some again cross the South Atlantic below the Equator to the New 
World side. This species thus spends its summers and winters in re- 
gions of continuous or near continuous daylight. It is truly said that it 
holds the record for all life in the amount of daylight seen. It also 
holds the record for distance, the extremes of its summer and winter 
homes being some 11,000 miles apart, which would mean perhaps 
25,000 miles of flight a year. The bird also holds the known record 
for distance of recovery, a chick banded in Labrador, July 23, 1928, 
having been recovered in Natal, Union of South Africa, November 
14, 1928, some 8,000 or 9,000 miles distant. 

Premig ration Events. Some of the events preceding migration, 
either spring or fall, have already been touched upon. But from the 
number of unknowns, it is obvious that a considerable amount of pa- 
tient field observation in well-known regions as well as distant lands 
still lies ahead. 


(The recognizable prelude to migration in many birds, particularly 
diurnal migrant)begins with flocking together in summer, near the 
end of the nesting periodAP a g e 237). In part this may be assembling 
because of favored habitat./ A recognizable nervous tension is evident 
in the early fall among migrant birds in the North and in early spring 
among northern birds wintering in the South. Similar happenings 
occur in tropical winter ranges of northern birds. This state of evi- 
dent tension can be taken as an indication of impending departure?) 

Observation of the southward departure of the Cliff Swallow near 
its northern limit has indicated some of the premigration events. 
Noticeable shortening of the day takes place by early August, though 
temperatures remain about the same except for greater coolness at 
night. Swallows not encumbered by care of young and the young on 
the wing increase their daily flight both in distance and time in the 
air, perhaps somewhat as an athlete prepares for a contest. A few 
waii^days may slow up this "preparation, 1 ' but a few cold ones will 
increase^ftemity. (The day before departure may be one of consider- 
able excitement, birds coming and going all day long with much twit- 
tering, often far into the night. On the morning of departure, flocks 
wheel about the sky in great sweeps, now and then taking off in a 
general southerly direction,}the exact compass bearing within the 
southerly direction depending upon valleys and ridges. By 2 hours 
after sunrise, none but nonflying young and their parents remain. 
A late-rising ornithologist might assume that the birds left during the 
night; dawn departure of many diurnal migrants may be more com- 
mon therefore than we suppose. 


25 301 5 10 15 20 25 I 5 10 15 20 25 301 5 10 15 20 25 301 5 10 

Allen Hummingbird 


Orange-crowned Warbler k jJUui I 
Pileolated Warbler J 

Tolmie Warbler JL Ji 

Lazuli Bunting a uH Jbi aJhi 

Western Wood Pewee . _ . Ji L 

Fig. 16* 10. Spring arrival dates of summer residents in the Berkeley, 
California, area (1911-1941). Solid block squares indicate first-seen rec- 
ords; half squares, probable early vagrants; open squares, doubtful first- 
seen records. (After Henry G. Weston, "Spring Arrival of Summer Resi- 
dents in the Berkeley Area, California," Condor, 50(1948) :8 1-82.) 


Spring Migration. Migration (both spring and fall) follows a 
definite order. Some species migrate early, some late, some in between 
(Fig. 16*10). A 5-year average at one place will generally show the 
arrival order of the various readily observed species (Table 16-1). 

Table 16-1 
Average Dates of Arrival of Swallows, Ann Arbor, Michigan 

Number of 
Years Reported 

Average Arrival 

Arrival Sequence 








Tree Swallow . 
Purple Martin 
Barn Swallow . 
Bank Swallow . 
Cliff Swallow . 
Rough- winged 

.. 15 
.. 14 
.. 17 
.. 14 


















Source: Based upon data from Norman A. Wood and A. D. Tinker, Fifty Years of 
Bird Migration in the Ann Arbor Region of Michigan, 1880-1930, Occasional Papers 
of the Museum of Zoology, University of Michigan, No. 280. 

On the basis of the few species studied in detail, it appears that in 
general, males arrive before the females and old birds before first-year 
ones. The order in 1911 for the Red- winged Blackbird at Ithaca, 
New York, follows (Allen, 1914): 

Vagrants: February 25-March 4 
Migrant adult males: March 13-April 21 
'Resident adult males: March 25-April 10 
Migrant females and immature young: March 29-April 24 
Resident adult females: April 10-May 1 
Resident immature males: May 5-June 1 
Resident immature females: May 10-June 11 

It seems entirely likely that studies of other species in other localities 
would be productive of similar results. The northward advance of the 
hardy birds follows close upon the departing heels of spring. Phoebes, 
Geese, and Mergansers in America may follow close upon the melt- 
ing of ice and snow. The Cliff Swallows move northward and north- 
eastward from their path through Central America and Mexico as 
they come up from South America (Fig. 16-11). The European 
migration of the Barn Swallow shows a parallel thrust along the west 
coast of Europe, where it averages about 35 miles a day. 

Spring and fall migration through the Mississippi Valley deserves 
special mention. Topography, vegetation, and wind in concert with 
position "funnel" migrating birds up and down the Mississippi Valley, 
which has been called the greatest migrating flyway on earth. No- 



where else will birds be found in such numbers over so wide an area; 
nowhere else do the birds pass by in such waves of moving life as in 
this great area extending from the Plains to the Appalachians. 

Fig. 16-11. The Cliff Swallow moves around the Gulf of Mexico. 
Being a diurnal migrant, it feeds as it flies. Though western records are 
few and migration in the West somewhat conjectural, isochronal lines 
have been drawn to indicate earlier arrival in the West and retarded ar- 
rival in the East. (From Frederick C. Lincoln, Migration of Birds, U. S. 
Department of the Interior, Fish and Wildlife Service, Circular No. 16, 
p. 17.) 


Fall Migration. Fall migration may start immediately after the 
young leave the nest, but some species may wait until later. Many 
Shorebirds return to the northern states from the Arctic in July, 
shortly after the last group going north has passed. Adults of some 
species desert the young and depart south from northern breeding 
grounds and let the young shift for themselves. Canada Geese, how- 
ever, tend to migrate as a family unit, while the Cliff Swallows mi- 
grate as a colony or part of a colony. Swallows tied down by care 
of young sometimes remain behind while other individuals of their 
species that have finished nesting move on. In colonial birds especially, 
some desertion by parents with weak family ties may take place. 

The sequence of species in fall resembles somewhat that for spring 
and the average for a 5 -year period is likely to be about the same as 
for other years. Early fall migrants may depart long before the com- 
ing of cold weather; the Blue-winged Teal migrates early, for ex- 
ample, often beginning in August. In the same way, the Bobolink 
starts southward about mid-August; it moves southeastward into the 
eastern Gulf states and continues across the Caribbean into northern 
South America, where the first birds arrive before the middle of Sep- 
tember. The winter home in northern Argentina and adjoining re- 
gions is reached sometime in November (Fig. 16-12). 

Migration and Winter Range. The habitat chosen throughout 
the year remains substantially similar for most birds. Birds of the 
brush migrate and winter in brush, while those of the open use open 
habitat. Some adjustment manifestly must occur, as in birds like the 
Warblers of the coniferous boreal forest that winter in broadleafed 
tropical regions. 

A bird that has once wintered in an area is likely to return again 
another winter, just as nesting birds return to their previous residence. 
Young raised in an area from eggs obtained elsewhere tend to return 
to the place of rearing, not where the mother laid the egg. The re- 
covery of banded nestlings from the same nest often shows them to 
have wintered far apart. A brood of European Widgeons banded in 
Iceland scattered far, some being killed in Europe and some in the 
United States. ' While this is a case involving exceptionally long dis- 
tances, the recovery of many brood-mates indicates clearly that they 
tend to scatter. But we are ignorant of the system involved, if any. 

The fixity of both winter and summer range, when once estab- 
lished, is important and may bear upon problems of the bird's life. 
No doubt return to familiar territory is an advantage in winter as well 
as in summer. It seems likely that some entire local populations may 
migrate and winter together. Geese migrate back and forth as family 
units and probably community groups also. Their exact path and 



Breeding range 
Winter range 

Fig. 16-12. Distribution and migration of the Bobolink. It is assumed 
that birds in the western colonies, established since the coming of the 
'white man (circles on the map), migrate east and south rather than by 
taking the short cut through Mexico. (From Frederick C. Lincoln, Migra- 
tion of Birds, U. S. Department of the Interior, Fish and Wildlife Service, 
Circular No. 16, p. 56.) 



respective ranges are influenced by tradition. Because the families 
stay together, those that know the way are accompanied by young 
unfamiliar with the route and range, so that over the years a fixity of 
route may result. This influence of tradition may explain the shift 
reported in the Snow Goose route in 1884 from the eastern to western 
shores of Hudson and James Bays. Wind is said to have forced the 
Geese across James Bay, and the route is still followed. 

In Europe, the general direction of migration is to the south and 
southwest, though some birds move in a south-easterly direction into 
southern Asia. Some have also been reported migrating nearly east- 
west. Birds of adjoining continental areas often pass into the British 
Isles. Birds of Europe that winter beyond its borders tend to cross 
into Africa, some to South Africa. The migration in Asia is less 
known, but it seems to be south and southeastward into adjoining is- 
lands and the lands of the Pacific, or southwestward into Europe and 
Africa. Some species may reach Australia, while many winter in 
Asia Minor or pass on into Africa. 

A few birds from Alaska cross into Asia to winter, just as the 
Arctic Tern may cross to the east side of the Atlantic (Fig. 16-9). 

Fig. 16-13. The White-throated Sparrow appears in the West only as 
a straggler. In winter, its zone of concentration lies in the Southeast. The 
isopleth lines indicate average birds reported per hour of cemusing during 
the Audubon Christmas Census, 1900-1939. 


These are explained as migration routes that retrace the ancestral path 
by which the species spread. 

Concentration Areas! v Birds may winter over a great area but 
concentrate in only part of it where conditions are most favorable.) 
The White-crowned Sparrow winters over much of the United 
States, but its winter distribution shows a concentration area in the 
West. The White-throated Sparrow appears in the West as a strag- 
gler and has a concentration area in the Southeast (Fig. 16*13). The 
Tree Sparrow winters over much of the United States and Southern 
Canada, but its concentration area lies in the mid-south. The Ameri- 
can and Red-breasted Mergansers winter wherever open water can 
be found. But the Red-breasted is more coastal and its concentration 
area not so far northward. 

The migrations of a few North American birds illustate the rela- 
tionship of summer and winter range. The Harris Sparrow breeds in 
northern Canada and winters in the south-central region from Ne- 
braska to Texas. Fall stragglers, largely young, may wander west to 
California or east to Ohio. The subspecies of the Fox Sparrow along 
the coast of western North America have long been noted for their 
use of "leap-frog" winter range. The races living farthest north 
winter farthest south and other races winter in between. 


SCHMIDT, Principles of Animal Ecology. Philadelphia: W. B. Saunders Co., 1949. 
*ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 

*ALLEN, GLOVER M., Birds and Their Attributes. Francetown, N. H.: Marshall Jones 

Co., 1925. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
CLARKE, WILLIAM EAGLE, Studies in Bird Migration, 2 vols. London: Gurney & 

Jackson, 1912. 
*GRISCOM, LUDLOW, Modern Bird Study. Cambridge, Mass.: Harvard University 

Press, 1945. 
HEAPE, WALTER, Emigration, Migration, and Nowadisn. Cambridge, England: W. 

Heffer & Sons, Ltd., 1932. 
"LINCOLN, FREDERICK C., The Migration of American Birds. New York: Doubleday, 

Doran & Co., 1939. 
LINCOLN, FREDERICK C., Migration of Birds. U. S. Department of the Interior, Fish 

and Wildlife Service, Circular No. 16, 1950. 
MATTHEWS, G. V. T., Bird Navigation. Cambridge, England: Cambridge University 

Press, 1955. 

PEARSE, A. A., Animal Ecology. New York: McGraw-Hill Book Co., Inc., 1939. 
"THOMSON, A. LANDSBOROTJGH, Bird Migration. London, England: H. F. & G. 

Witherby, Ltd., 1936. 

WETMORE, ALEXANDER, The Migration of Birds. Cambridge, Mass.: Harvard Uni- 
versity Press, 1927. 
SIMMS, ERIC, Bird Migrants. London: Cleaver-Hume Press Ltd., 1952. 


Bird Song 

Rivaling bird flight as the most distinctive single characteristic of bird 
life, bird song reaches a state of perfection altogether unknown among 
any group of animals. It is, paradoxically, one of the least studied at- 
tributes. No invertebrate group or other vertebrate group has song 
so widely used and so well developed. Because birds possess the very 
great mobility that flight confers, sound, with its high speed of trans- 
mission, seems a better mode of communication than scent, which is 
slow. Animals of low mobility find scent suitable, either alone or in 
conjunction with modest vocal powers. 

Among the vertebrates man alone seems to have a musical dis- 
crimination at all like that of birds. The reasons may be the same, 
chiefly that both man (with a poor sense of smell) and birds depend 
mostly upon sight and sound for detecting friends and foe alike, or 
for judging the state of the environment. The fact that birds use 
musical elements similar to those of human music suggests that (a) 
one learned from the other, (b) both accidentally struck upon the 
same pattern, or (c) music as we know it is a deep-seated, biological 
attribute. Because some frogs and some mammals also use musical 
notes as of the "human" musical scale, an ancient origin of music in 
animal history is indicated. 

Studies show that from the psychological standpoint, the ear re- 
ceives and becomes aware of patterned sound more easily than un- 
patterned sound. It is not clear, however, whether patterned sounds 
have greater radiating power than unpatterned ones or whether they 
may be produced more easily. Yet the evidence that sounds may be 
received and understood more easily when patterned indicates that 
communication (as measured by the effort at reception on the part 
of hearers) in turn is more efficient when effected by patterned sound. 
Music is patterned sound, and its adoption becomes more understand- 




able against the background of the struggle for use of biological 
energy. It indicates quite clearly that music is yet another animal char- 
acteristic of an adaptive nature. 

Birds produce vocal sounds in the syrinx, a structure peculiar to 
them, whereas mammals produce vocal sounds in the larynx. The 
mammalian voice box with which so many people gain familiarity 
through use consists of a cartilaginous enlargement at the upper 
end of the trachea termed the larynx (and commonly called the 
"Adam's apple"). This structure contains vocal cords, thin bands of 
fiber sheathed in a mucous membrane. The tension and placement 
may be altered by means of muscles. At will, the column of air passing 
across the vocal cords, usually during exhalation, causes them to move 
and to generate audible vibrations. Birds, on the other hand, possess 
no functional larynx. The syrinx is situated, not at the upper end of 
the trachea as in mammals, but at the lower end of the trachea. 


Syrinx: The Voice Box of Birds. Birds have three kinds of syrinxes 
(also spelled syringes): tracheal, bronchial, and tracheobronchial 
(Fig. 17*1), all of which bear a general similarity. Some anatomists 
distinguish only the second and third types; confusion apparently 
stems from the fact that some anatomists consider certain elements as 
split tracheal which others consider as split bronchial rings. The 
syrinx demonstrates yet again the tendency of bird evolution to trans- 
fer weight to the body interior. If a muscular larynx were at the 

Fig. 17-1. Three kinds of syrinxes: (a) Tracheal, (b) bronchial, and 
(c) tracheobronchial; t.c., tracheoclavicular muscle. (After Alfred New- 
ton, A Dictionary of Birds, p. 940. London: Adam & Charles Black, 


upper tracheal end, for example, its weight and that of associated 
tissues would be brought ahead of the wings. The syrinx at the lower 
end brings weight near the body center and under the supporting 
wings. The long trachea carrying sound also offers possibilities for 
voice modification not otherwise so easily obtained. 

The tracheobronchial syrinx (the most common kind), includes 
modifications of the two bronchi and tracheae, as the name implies. 
The anterior bronchial rings are incomplete on the dorsal side, leav- 
ing a gap closed by the tympanic membrane. The pessulus extends 
across the bronchial rings and is covered by the semilunar membrane. 
The sounds are produced by vibration of the tympaniform membranes 
in many and perhaps in all birds. In the bird (at least in the Passerines) 
it appears that air passing outward (i^e., exhalation) makes all sounds 
(Miskimen, 1951). Intrinsic and extrinsic muscles (of the interior 
and exterior of the syrinx) control the shape and tension of the cham- 
ber, pessulus, and semilunar membranes to produce variations of 
sound. The tenth and twelfth cranial nerves innervate these muscles 
(page 74). 

The tracheal syrinx resembles the tracheobronchial syrinx in con- 
struction but involves only the trachea, and even then only its lower 
part. It occurs in Wood-hewers and Ant-thrushes of South America 
and perhaps also is the type found in Storks. 

The bronchial syrinx is formed of incomplete rings and loose mem- 
branes in the bronchi somewhat similar to those of the tracheobron- 
chial syrinx. The passage of air sets up vibrations in the loose mem- 
branes between the bronchial rings. This type of syrinx occurs in 
some and perhaps in all members of the Cuckoo, Goatsucker, and 
Owl families. 

The general make-up of the syrinx thus consists of (a) the sup- 
porting framework, (b) vibratory internal membranes of several 
types, (c) muscles for controlling the membranes and syrinx, and 
(d) nerves for controlling the muscles. Singing ability depends upon 
muscle and nerve abundance, as well as upon the excellence of co- 
ordination. It reaches its highest state in the Passerines, which may 
have five to seven pairs of syringeal muscles. The vibrations of the 
syringeal membranes set up vibrations in the column of air. The 
length of the trachea seems to influence the tone largely by acting as 
a resonator or modifier. 

Modifications and Accessory Organs. Some birds have modifi- 
cations of the vocal organs or have accessory organs for controlling 
or assisting in the production of sounds characteristic of the species. 
Both the Trumpeter and Whistling Swans, for example, have greatly 
lengthened windpipes (tracheae) for modifying or amplifying the 


tone, somewhat as does a trumpet. These windpipes actually invade 
the sternums. The trachea of the Whistling Swan has a simple extra 
convolution, but the Trumpeter Swan not only has an extra convolu- 
tion but also a dorsal bend in the windpipe that fills a projection from 
the sternum into the body cavity. 

The Whooping and Sandhill Cranes have tracheae likewise enter- 
ing the hollow keel of the enlarged sternums. The enlargement in the 
Whooping Crane includes some 27 inches of the coiled trachea, 'itself 
about 50 inches long and actually longer than the body of the bird. 
The extra convolution of the Sandhill Crane windpipe is but 8 inches 
compared to 27 inches for the Whooping Crane. The respective 
names of Whooping Crane, Whistling Swan, and Trumpeter Swan 
reflect the differing voices associated with the differing tracheal con- 
volutions (Fig. 17-2). 

(a) (6) 

Fig. 17*2. Coiling of the tracheae of the Whooping Crane (a) and 
Sandhill Crane (b) in the stermnm. The tracheae actually coil in a some- 
what spiral direction so that parts of the loops in these views are behind 
others. (After Elliott Coues, Key to North American Birds, p. 209. 
Boston: Dana Estes Co., 1903.) 

Paired resonating chambers are common among members of the 
Tetraonidae. In the male Sage Grouse of the American sagebrush 
country, the lateral walls of the esophagus can be distended by mus- 
cles having their origins in the bony framework of the neck and their 
insertions in the esophagus. The performing bird apparently closes 
the trachea at the glottis (possibly the nostrils also), and pumps air 
from the lungs into the distensible pouches. Careful field observation 
indicates their capacity, for it requires three intakes of air exhaled into 
the esophagus to fill them (Scott, 1942). At the conclusion of the 
picturesque display, the sudden collapse of the pouch and escape of 
air makes a resounding PLOP, reportedly as the sides of the pouches 

Resonating chambers are found also in the Prairie Chicken, Sharp- 
tailed Grouse, Black Grouse, Capercailie, Spruce Grouse, Franklin 
Grouse, and Blue Grouse (Figs. 17-3, 17-4). These evidently func- 



Fig. 17*3. The inflated resonating chambers of the Prairie Chicken 
show in the "booming" bird at the right and the one pausing at the left. 
The feathers of the throat open to reveal the bright yellow pouches. 
(Photograph by Staber W. Reese. By pennission of the Wisconsin Con- 
servation Department.) 

Fig. 17*4. The courtship of the Sharp-tailed Grouse includes dancing. 
Vocal duckings are part of the performance, during which the neck 
pouches are inflated. (Photograph by Staber W. Reese. By permission 
of the Wisconsin Conservation Department.) 


tion to build up resonance. It may be that the actual passage of air 
across the opening of the pouches produces the sound or adds res- 
onance. The Emu has an air pouch also, a single one opening off the 
lower side of the windpipe at a point where the rings are incomplete. 


Instrumental Calls and Songs. Some birds have nonvocal or 
"instrumental" calls and songs either replacing vocal ones or adding 
to them, but most birds using "instrumental" calls or songs have rather 
low vocal ability. There are five general ways of producing nonvocal 
sounds; these involve (a) escape of air, (b) use of feet, (c) use of 
wings, (d) use of tail, and (e) use of bill. It has been said that all 
instrumental sounds of an orchestra have their counterparts in various 
bird sounds (Armstrong, 1947). 

The PLOP! made by collapse of the air pouches in the Sage Grouse 
has already been mentioned. The Ostrich and some Geese produce 
hissing sounds by the passage of air through the mouth. Even the 
Black-capped Chickadee incubating in its nest hole will give an explo- 
sive hiss at an intruder. 

A few birds produce sounds by stamping their feet in a dance. 
Prairie Chickens dance in this way during the courtship performance 
as a prelude to bootmng^ but their stamping makes a rather weak 
sound heard only a short distance. It is subordinate to the booming 
proper (Fig. 17-3). A near relative and associate, the Sharp-tailed 
Grouse, depends largely upon its feet for making sounds in courtship. 
(B^it some observers have suggested that the sound results from rat- 
tling of the wing or tail feathers). The males stamp their feet with 
great rapidity and seem to "float over the ground like drops of 
water on a hot stove," meanwhile producing loud sounds having the 
rhythm and likeness of those from an air drill or riveting hammer 
(Fig. 17-4). 

The male Ruffed Grouse mounts a log or other platform and drums 
with his wings; he produces sound by the quick snap of his wings 
against the air (Fig. 17-5). The common barnyard rooster also pro- 
duces a flapping sound by drumming with his wings; even a male 
Ring-necked Pheasant on his crowing grounds drums with his wings. 
Drumming flight occurs among several members of the Grouse family. 
The Blue Grouse, for example, rises a short distance into the air and 
drums rapidly with his wings as he rises (about 3 feet off the ground) 
and descends (Wing, 1946a). 

The peculiarly constructed outermost primary feathers of the 
Woodcock make a whistling sound in flight. The birds feed and 




fly much at night and the whistling of the wings may be both a warn- 
ing sound and a position marker. If the outermost primary of each 
wing is clipped, the whistling sound will not be produced. The tail 
feathers of the Anhinga have a "fluting" that gives them a washboard 
look; the fluting at times makes a rippling sound as the bird moves 
through the air. The ivinnoiving of the Snipe as it circles over the 

Fig. 17*6. The characteristic position of the Snipe and its tail in mak- 
ing its "bleating" sounds during flight. (After P. H. Bahr, "On the 'Bleat- 
ing' or 'Drumming 1 of the Snipe (Gallinago coclestis)," Proceedings of 
the Zoological Society of London (J901):l2-35.) 

marsh is caused by air vibrating the outer tail feathers especially de- 
veloped for this purpose (Fig. 17-6). The Manakin of the Neotropi- 
cal region makes a rattle with its peculiar wing feathers as it jumps 
back and forth through the air. Only the male has these special 
feathers (Fig. 17-7). 

Function of Songs and Calls. There seem to be three general 
functions of calls and songs in the bird world: communication, ad- 
vertisement, and identification. Advertisement and identification per- 
haps are merely forms of communication, so that fundamentally 
sounds have but a single function (unless they serve for the satisfac- 
tion of the bird itself or as outlets for the emotions, as has been sug- 
gested in regard to some bird songs). 

A simple example of communication is the call of a lost chick and 
the hen's clucking response in the farmyard. But communication calls 
need not be directed, for the call of the same lost chick seized by a 



person or enemy is a cry of fear or alarm understood by the bird 
world in general, even though the mother herself is the one most likely 
to respond. Many species recognize the hunger call of young birds 
not or their own kind. 

Calls between members of a mated pair (e.g., Fig. 19-5), such as 
during interchange of incubating birds, may be identification calls: 
they may also signal readiness to change places; and they may^be part 
of a complicated behavior pattern. Birds that flock have calls thai 
serve to keep the flock in contact; thus, the continuous twitter of the 
Slate-colored Juncos in a flock seems to aid materially in maintaining 
the flock continuity. Lost or detached members of a flock are guidec 
back by the flock calls as well as by responses to their own "lost-bird' 
call notes. Flock calls and twitters not only serve to maintain flocl 
continuity but may serve also as sequestration notes that maintair 
distance between the various members within the flock. 

The 'warning call of the American Robin or Song Sparrow 
whether given in the flock or when a bird is alone, is recognized bj 
other Robins and Song Sparrows and many other species as well. 

Vocal sounds often serve as an important activity and behavioi 
regulator of Black-capped Chickadees, especially in the flocking 
season. The use of sounds made by this species has been particularl} 
well explored. It is known that at least sixteen different calls and note; 
are used, eight of which function primarily during breeding behavioi 
and eight primarily in general social relations, as follows (Odum 
1941-1942): "Phoebe" song, signal song, alarm note, recognition note 
contact note, flight or restless note, warning note, dominance note 
musical "to- will," begging note, mating (?) note, "perplexed" note 
"hissing" or "bluff" note, distress call. Comparable studies of othei 
species may show similar "language." 

Sounds may have several functions at various times (Armstrong 
1947). They may serve to: 

1. Identify the singer to its own or other species 

2. Place-mark the bird and its territorial holdings 

3. Indicate the vigor and dominance of the singer 

4. Indicate the stage of the singer in the breeding cycle 

5. Indicate location of the bird in relation to communal roost 

6. Induce another bird to disclose its sex 

7. Attract the sexes and influence sexual behavior 

8. Intimidate and drive off intruders 

9. Provide signal for activity change, as at the nest 

The very great individual variation in songs so characteristic o 
some species suggests a functional purpose. But of this we are igno 
rant. The variation among Song Sparrows and Eastern Meadowlarks 



for example, may indicate that they have a greater need for individual 
recognition or identification by ear than do birds that sing the same 
few notes over and over as in the Black-capped Chickadee. But there 
may be variations of quality or expression in the seemingly simple 
songs that accomplish much the same thing. 

Singing Position. Birds tend to sing from conspicuous perches, 
even though customarily they may inhabit the interior of bushes or 
dwell on the ground. Birds normally inconspicuous or retiring, like 

Fig. 1 7 8. Flight song of the Venmlion Flycatcher. The bird hovered 
in air while singing the song, shown by its musical notes, at each 
wavy place in the line tracing the flight path. Arrows indicate the direc- 
tion of flight and final dive into a red cedar tree. The song was repeated 
thirty -nine times before alighting. (Drawn from life by Anne Hinshaw 
Wing, at Junction, Texas, June 30, 1950.) 

the Grasshopper Sparrow, may mount to an exposed perch at the top 
of a tree, bush, or weed to sing. Some ground-nesting birds, like the 
Savannah Sparrow and Meadowlark, ascend low natural perches and 
fence posts if available. Several Vireos have the interesting habit of 
singing while on the nest. 

It would seem to be an easy step from using an open song-perch to 
singing on the wing (page 283). The Skylark of the Old World has 
long been known for this habit, but many birds in the New World 
do likewise. Actually, flight song is a somewhat characteristic habit 
among birds of the grassland, prairie, desert, and marsh where perches 
are few. Some of these birds have been called lark in recognition 


of singing flight. Among the best known birds engaging in singing 
flight are the Snipe, Nighthawk, Ptarmigan, Pipit, Vermilion Fly- 
catcher, Scissor-tailed Flycatcher, Chat, Lazuli Bunting, Horned 
Lark, Lark Bunting, Meadowlark, Bobolink, and American Marsh 
Harrier of the open fields, grasslands, pastures, or marshes (Fig. 17-8). 
Yet some birds of brush and woodland engage in singing flight at 
times, occasionally over the forest canopy, and sometimes often 
enough for it to be called a subordinate habit. The Ovenbird and 
Boat-tailed Crackle, for example, sing in flight even though this is not 
so pronounced nor so spectacular a habit as in birds of the grassland 
like the Lark Bunting. Many birds like the Mockingbird, Painted 
Bunting, and House Finch may continue singing during flight from 
one perch to another, but their songs may or may not be true flight 
songs. In other parts of the world, flight song occurs also. Among 
the examples may be listed the Whitcthroat, Tree Pipit, Rock Pipit, 
Glossy Grassquit, King Bird-of-paradise, Pratincole, Greenshank, 
Lyre-tailed 1 loncy Guide, Dunlin, and Black Penelope (see also 
Courtship Performances, Chapter 18). 

Calls of Adult and Young. Males do most of the singing during 
the breeding season, and the syrinx of the male often shows a greater 
development than that of the female. Some immature male birds may 
sing, but usually only the mature males do so. The first-year bird's 
song may be less complete than that of the older male at the start, 
which suggests the need for practice or listening to others and prob- 
ably also for further physical and nervous development. Some birds 
still wearing immature plumage may sing about as well as the adults; 
this is especially noticeable in the American Redstart, Purple Finch, 
House Finch, Painted Bunting, and Red Crossbill. These species, 
however, may take more than one season to acquire the fully adult 

Because singing by males is so apparent, singing by females has 
been generally overlooked; yet females of some fifty-six species are 
known to sing (Nice, 1937, 1943). Call notes of females generally 
are the same as those of the males, although as in man, the voice of the 
female may perhaps be softer and higher pitched. The Horned Owl 
and other Owls show this characteristic, which may indicate a trait of 
the order (Miller, 1934). The call of the female may vary in tone 
quality and loudness in the Mallard Duck and other dabblers. The 
males of Dabbling Ducks have a softness of voice produced by a 
muting device in the syrinx, which itself is actually larger and better 
developed than that of the loud-voiced female. The singing of fe- 
males may be (a) rather indiscriminate calls associated with the song 
of the male, (b) antiphonal response to the song of the male, (c) 


sounds fitting into those of the male to make a "joint song," or (d) 
sounds independent of any by the male. 

Bob-white young hatched in an incubator will sing the typical 
"Bob-white!" when adult, although they have never heard any such 
song before. But the young of species having more complicated songs, 
like the Meadowlarks, native Sparrows, and others, may possibly have 
to learn the finer points of their song by hearing others sing it, though 
of this subject little is known. The first hearing of the adult song may 
release a pent-up channeling of impulses, so that from then on the 
true song will be sung as in the species. Some ornithologists have sug- 
gested that among some species the male sings near the nest during the 
late stages of nesting so that the young can hear the song of the spe- 
cies, the memory of which it retains until it becomes an adult, or may 
even reinforce by singing when it is still an immature bird. The evi- 
dence for the development of song and its relation to instinct and as- 
sociation is rather meager (Thorpe, 1951). 


Songs and Seasons. Bird song reaches its highest state of per- 
fection during the peak of the breeding season. Karly singing occurs 
on warm days in cool regions. Temperatures lower than 50 F. in- 
hibited Song Sparrow singing in mid-January; by mid-February, the 
inhibiting limit had dropped to about 28 F. (Nice, 1937). Karly 
songs in northern latitudes may show incompleteness. In mixed flocks 
of Prairie and Northern Horned Larks found together in the same 
field, the Northern Horned Larks can be differentiated by their song, 
which has only its first few notes whereas the song of the Prairie 
Horned Lark is then the complete or nearly complete song character- 
istic of the species. Later in the season, Northern Horned Larks will 
also sing the complete song. Early songs usually occur first on warm 
spring days; however, some birds may sing on sunny days in mid- 

The autumn recurrence of drumming in the Ruffed Grouse has 
been termed fall recrudescence. It may be a premature expression 
from an early hatched bird or perhaps a contrastimulation from 
gonadal refraction. Snatches of song also occur among early hatched 
birds but the renditions are usually incomplete. Other premature acts 
of immature birds such as nest construction and northward fall move- 
ment (page 302) have been reported. The significance of the fall 
recrudescence, however, has not been determined. 

Length of Singing Seasons. Singing declines rather slowly with 
the passing of the height of the breeding season in mid-latitudes but 




AUGUST 15 - 



JULY 31 

N CO <T> Q 
cvj c\J c\j i*) 
CD 0) <7> <J> 

Fig. I7 9. /4 14-year record of the cessation of song by the Hermit 
Thrush and American Robin. (After Aretas A. Saitnders, "The Seasons 
of Bird Song: The Cessation of Song After the Nesting Season" Auk, 

Table 17-1 
Song Order and Season for Ten Species in Arkansas 






(Ending Season 




Mourning Dove 

March 1-Scptenibcr 1 




Eastern Phoebe 

March 10-October 1 




\Vood Pewce 

May 1-Scptcrnber 10 




Bell Vireo 

May 5-September 5 




Yellow-throated Vireo . . . 

April 10-September 15 




Black and White Warbler . 

April 1-July 25 




Yellow Warbler 

April 18-July 10 




Yellow-throated Warbler. . 

April 15-August 10 




American Goldfinch 

March 20-October 20 




Red-eyed Xowhee 

February 10-August 15 




Source: W. J. Baerg, "The Song Period of Northwest Arkansas," Auk, 47(1930): 


with dramatic suddenness in the Arctic regions where daylight de- 
clines rapidly after midsummer. In the Northland, song comes to a 
rather abrupt end in a matter of days, whereas it may take weeks 
farther south. The cessation of song in summer and fall is less uniform 
in mid-latitudes than is the beginning of song in spring. There may be 
a month or more of difference between the dates of ending in succes- 
sive years. Part of this may result from difficulties that observers are 
bound to have recording the exact date of last song. Some confusion 
may result from the fall singing of immature birds, such as the imma- 
ture Yellow-throated Vireos that may sing well before the fall mi- 
gration (Sutton, 1949). Fig. 17-9 shows a 14-year record of song ces- 
sation. Table 17-1 gives examples of the song periods for ten species 
and shows the order of spring song-start and fall cessation as well as 
length of song season. 

Hours of Singing. Among diurnal birds, singing during the height 
of the breeding season reaches its peak in the morning. As a general 
rule, singing begins near dawn and rises to a peak a few hours after 
sunrise, followed by a lull during the middle of the day and a resump- 
tion toward evening (see Fig. 11-5). It thus parallels the daily ac- 
tivity rhythm (page 213). Karly in the season, song may be rather 
uniformly distributed throughout the day, but as the season pro- 
gresses, singing occurs chiefly in the early morning and evening. 
Midday singing occurs regularly early in the season but later only on 
cool or cloudy days; in fact, there is a marked resumption of singing 
on cloudy days. Even the passing of a heavy cloud (or eclipse) may 
bring outbursts of song in the American Robin. Birds are attuned to 
light conditions, the amount of light being relative (see Chapters 1 1 
and 16). Late in the season, birds sing only in the early morning or 
late evening hours and stop during the heat of the day, although there 
are exceptions, such as the Warbling and Red-eyed Vireos, that sing 
more or less continuously even in midsummer. 

Nocturnal and crepuscular birds, like the Screech Owl and Whip- 
poor-will, sing in dusk and darkness; they rarely sing at all during the 
day except when it is cloudy. Yet some diurnal birds may sing at 
night. The Mockingbird is especially likely to sing on moonlight 
nights and often on dark nights as well. The Ruffed Grouse may 
start drumming early in the morning, several hours before daylight, 
and may even drum irregularly at other hours during the night. In 
the high latitudes, some birds sing many hours a day. The Gambel 
Sparrows, for example, stop all singing during only an hour or so in 
the Arctic night a twilight sometimes in actuality. The Gambel 
Sparrows may sing at intervals during about 22 hours of the 24; when 
they sleep is not quite clear. 


Repetition of Song. Songs vary widely from the short, simple, 
single note of the Pygmy Owl, for example, to the long complex 
series of musical notes of the Winter Wren. It is said that, in general, 
the shorter the song, the greater the likelihood of its immediate repeti- 
tion, and that the longer songs come at longer intervals. But there 
are many exceptions; the Pygmy Owl sings at rather long intervals, 
longer than those of the Winter Wren. Because sound as a mearis of 
communication does not linger long (compared to the lasting quali- 
ties of scent used by animals which use the nose for gathering mes- 
sages) the need for repetition of song seems real and practical. It is 
reported also that a monotony threshold exists. Singing of repetitious 
songs usually is discontinuous (e.g., Prothonotary Warbler); contin- 
uous singing usually is versatile (e.g., Thrushes) (Hartshorne, 

A Chuck- will's-widow sang, by actual count (Johnson City, Texas, 
June 28, 1950), at the rate of 27 songs per minute at 9:20 P.M. and 
of 23 per minute at 4: 25 A.M. the next morning. As these two periods 
are at the peaks of diel activity for a nocturnal bird, even with half 
as much singing during the "lull" period the bird would have sung 
10,000 times a night. Its 100 days of singing would suggest a million 
songs, even though the count indicated was near the end of the song 
season. The song of the Chuck-willVwidow has a regular five-note 
length not counting the opening chuck. The song of the Whip-poor- 
will has three notes (others may be heard sometimes at close quarters) 
and it has been estimated to sing at the rate of 58 to 63 songs a minute. 
The song of the diurnal Chipping Sparrow, on the other hand, has 
from 10 to 18 notes. A careful count indicates that 12 to 15 notes with 
an average of 1 3 is usual. The number of songs averages 5.5 per min- 
ute during the peak of singing. Presumably no fewer than 200,000 
songs would be sung in the 150-day season, and probably a figure 
twice that would be more nearly representative. Its relative, the Clay- 
colored Sparrow, sings 7 to 10 times a minute. 

The Blue Grouse sings ("hoots") a series of 4 to 6 notes, but fully 
90 per cent of its songs contain 5 notes. More than 50,000 songs may 
be sung during the season. The Ruffed Grouse may drum 10,000 
or more times in the spring. But the Bob-white whistles only during 
bachelorhood; one favored by early mating might whistle as few as 
100 times in a season. An American Redstart sang intermittently at 
a rate of 14 songs a minute. It seems probable, however, that among 
diurnal North American singers, the Red-eyed Vireo (35-41 phrases 
a minute) or one of its fellow Vireos may hold the record for the 
greatest singing effort during the season. They sing rather regularly, 
even during the middle of the hot summer days when all else is still, 


almost to the end of the song season. In its 130-135 days of singing, 
a Red-eyed Vireo may sing two to three million phrases ("songs"). 
A faithful ornithologist counted all the songs sung by a male Song 
Sparrow, which totaled 2,305 on the day of counting, May 11. The 
bird averaged 242 per hour in the morning and 150 in the afternoon 
(Nice, 1937). His top speed of singing seems to have been at the 
rate of about 5 a minute. Other reported song rates, converted to an 
hourly basis, follow: Red-eyed Vireo, 2,200; Eastern Phoebe, 2,100; 
Black-headed Grosbeak, 1,300; Clay-colored Sparrow, 480; Song 
Sparrow, 300; Chipping Sparrow, 330; and Prothonotary Warbler, 
300- 360. 


Physics of Bird Song. Like all other sounds, bird song consists of 
sound vibrations known as frequencies. The more rapid the fre- 
quency, the higher the pitch, and the higher the pitch, the higher the 
song sounds to the car. Man hears from a low of about 16 vibrations 
a second to a high of about 18,000, but the hearing range and its com- 
pleteness vary with individuals and with age. No doubt psychological 
reactions as well as training and practice enter into hearing ability. 
Bird hearing may reach (after conditioning) as high as that of man 
but only as low as 50 (Schwartzkopf, 1955). It is suggested that 
opening of the mouth by the bird may interfere with its own hearing 
through tension on the ear drum; this does not seem to be so in mam- 
mals (Bray and Thurlow, 1942). 

The hearing range of birds and mammals in general appears to 
vary somewhat with the range of the voice so that the latter roughly 
indicates the expected relative hearing range. A bird with a high voice 
appears to have a hearing range higher than that of a low voiced 
bird. But it must be remembered that measurements indicate the 
range in terms of pure tones, which rarely occur in animal voices. 
A bird like the Starling, for example, that probably hears no pure 
tones below 700 vibrations a second could hear the hoot of a Horned 
Owl at 256 vibrations a second because of overtones. In the same 
way, a Warbler that would not hear the ordinary speech of man if 
given in pure tones could perhaps be aware of a conversation under 
his perch because of these same overtones. It seems to be a workable 
generalization that birds hear and recognize instinctively the sounds 
of their enemies (e.g., a Crow hears the hoot of a Horned Owl) on the 
one hand, and the sounds of their prey on the other (e.g., a Screech 
Owl hears the squeak of a mouse). The highest sensitivity is in range 
of a bird's own voice; among exceptions are Owls whose highest sensi- 


tivity is in range of mouse squeaks (Schwartzkopf, 1955). The hear- 
ing ranges of several birds have been determined (Table 17-2). 

Table 17-2 
Hearing Ranges Determined for Several Species 

Frequency % 

Species (Vibrations 

per Second) 

Canvasback 190- 5,200 

Great-horned Owl 60- 7,000 

Rock Dove 200- 7,600 

Horned Lark 350- 7,600 

Starling 700-15,000 

House Sparrow 675-11,500 

Snow Bunting 400- 7,200 

A sound heard by the ear as a single note may upon analysis prove 
to be several notes given rapidly (Brand, 1938). The Song Sparrow 
song lasting 2 to 2 ! / 2 seconds may have as many as 35 or 36 notes given 
at the rate of 15 to 17 a second. These notes of one-fiftieth of a 
second in duration are followed by a pause of a two-hundredth of a 
second, and so on. There may be rapid changes of pitch as well, too 
rapid for the human ear to detect. One or more notes sometimes may 
be started before another has ended. Perhaps such changes may be 
heard as a double tone or multiple tone whose pitch is difficult to de- 
termine. The average human ear is unable to separate rapid notes 
from one another. Similarly, the eye recognizes the sixteen or more 
pictures cast on the screen each second by the motion-picture pro- 
jector as a continuous scene. Yet we know that it is not continuous. 
Audiospectograph analyses of bird songs, e.g., Carolina Wren (Borror, 
1956), show that the song pattern consists of a number of short 
phrases. Presumably these are the "building blocks" of which bird 
songs are made. The ability to determine time and pitch varies in 
man * and probably in individual birds also. 

The average frequency reported for Passerines is about 4,280 vi- 
brations per second, which is above that produced by the violin and 
piano but within the range of the piccolo. The average frequency is 
highest among Warblers and may be a full octave above the piano. 
Sound engineers say that birds follow the general rule that the smaller 
the instrument, the higher the sounds produced. A frequency of 

* An ornithologist who recorded his inability to hear bird songs as he advanced in 
years found that he could no longer hear the Golden-crowned Kinglet and Brown 
Creeper after he reached 60, Cedar Waxwing after 65, and Black and White Warbler 
after 67 (Saunders, 1934). 


12,225 vibrations per second has been reported for the Black-poll 
Warbler and 9,500 for the Grasshopper Sparrow (Brand, 1938). But 
whether or not microphones faithfully pick up the highest notes 
of birds remains to be seen. A sample of the frequency range reported 
for some birds appears in Table 17*3. How birds achieve pitch dis- 
crimination is a mystery (Schwartzkopf, 1955). 

Table 17-3 
Reported Song Frequencies for Some Birds 

Vibrations per 






Eastern \Vood Pewee 

. .. 4,125 



Black-capped Chickadee 


American Robin 


Hermit Thrush 

. . . 3,000 


. .. 3,475 


. . 2,600 

Kastern IVIeadowlark 


Western Meadowlark 

. . . 2,500 


. .. 2,800 

Song Sparrow 

. . . 4,700 

Source: Albert R. Brand, "Vibration frequencies of Passerine Bird Song," Auk, 

Music of Bird Song. Not only are there differences in the ability 
of different people to hear different pitches, but also in individual 
understanding of what is actually heard. Although most bird songs 
are generally considered pleasing, some are definitely musical in the 
human interpretation of the word. That is, some bird songs contain 
musical tones arranged in the form of brief melodious phrases. Not all 
listeners are able to distinguish between songs that are musical in a 
general way and those that are musical in the way that our own music 
is musical. 

Bird songs are esthetic expressions the medium of which is sound. 
They are rhythmic in the way that poetry is rhythmic. Many bird 
songs contain musical tones that can be recognized with more or less 
ease by persons with musical hearing. Yet even musicians may need 
to listen intently over a considerable period of time before it becomes 
clear to them what tones are sung and in what order. Some songs 
are intricate, high-pitched, and rapid to the extent that the aid of the 
physicist may be necessary to decipher them. The ornithologist, the 
physicist, the recorder of bird voices, and the musicologist need to 
work hand in hand for a complete understanding of the more difficult 
songs. Each type of research is important. 



Describing Bird Songs. A number of different systems have been 
devised to help people to recognize birds by their songs. No one 
system has been devised that is satisfactory for all songs or for all 
people. Songs are often described by means of syllables, catch- 
phrases, or sentences that suggest the sounds. The Eastern Towhee 
does seem to sing "See Tow Hee-e-e" or "Drink Your Tea-e-e" and 
the White-throated Sparrow seems to sing "Old Sam Peabody, Pea- 
body, Peabody." Diagrams are often found helpful, demonstrating 
the typical patterns of rhythm, pitch, syllabification, volume, and 
so forth. Verbal descriptions likewise shed light upon the nature 
of songs. Combination methods have been devised by some 

Duration In Seconds. 

1 2 */s3 

About 3 seconds tremolo et accelerando 


Fig. 1 7 1 0. The "Sounder's diagrams" use horizontal lines to indicate 
notes and their length,' vertical spacing indicates pitch; heaviness of the 
lines shows londness. The Field Sparrow song diagrammed on the left 
has been transferred to the musical scale on the right. 

There are so many different aspects of bird song that perhaps the 
best suggestion is that we go into the field and study them at first 
hand. Musically trained listeners may wish to use musical notations to 
describe the melodious aspects of bird song where these are present; 
those who are not musically inclined, however, had best use one of the 
other methods of description. The "Saunder's diagrams" have proved 
useful for many listeners to bird songs. In many cases, the songs so 
diagrammed may be readily transferred to the musical scale (Fig. 

The song of the Yellow-billed Cuckoo provides an example of the 
sort of song that is unsuitable for description by means of musical 
notation. This song depends for its esthetic effect upon voice quality, 
range, rhythm, syllabification, changes in velocity and volume, and in 
pitch. This is poetry rather than melody, "wordless poetry," or "tone- 
less music." 

To be truly musical, as our own music is musical, the songs of 
birds must contain musical tones. The essentials of music are said to 
be rhythm, melody, and harmony. Yet music need not contain har- 
mony as we use it today. It need be only melody. Melody is very 


old, but harmony that is, harmonious tones that are sounded simul- 
taneously has been used by man for his enjoyment only in recent 
centuries. Yet the best folksong has always been founded on an 
unconscious feeling for the principles of harmony. 

Music is natural rather than invented or discovered by wan. Music 
is composed of musical tones of a given key successively (and some- 
times simultaneously) sounded in a rhythmical way. It must be pleas- 
ing, according to the definitions. Both the selection of tones within 
the octave and their arrangement have physical and mathematical 
bases. Birds as well as human beings are intuitively aware of music 
and some in both groups are able to use it for their own purposes. A 
more limited use of musical tones is found elsewhere among living 
creatures, for example, in the utterances of frogs and toads. 

Although most songs are musical in a general sense or in a poetic 
sense, not all contain melody. One of the simplest songs that is musical 
in the melodic sense is the whistled "phoebe" or "feebee" song of the 
Black-capped Chickadee (Score la, \b, Fig. 17-11). It contains two 
tones of the musical scale in musical relationship. The song of Score 
la contains a melodic interval called a minor third-, Score Ib contains 
a minor second interval. The song of the Eastern Meadowlark in 
Score 2 is a brief melodious phrase, idea, or subject. A song of the 
Western Meadowlark, shown in Score 3, contains melodic intervals 
and also a harmonic interval, uncommon in bird song. 

Few birds are known to sing harmonic intervals or chords, although 
the study by physicists of recordings may provide us with additional 
species that do so. (The term "harmonic interval" is a musical term, 
not to be confused with the term "harmonics" used by physicists.) 
One of the birds whose songs contain chords is the Wood Thrush. 
This singer of the woodland can sound as many as four tones simul- 
taneously. The listener may gain a fleeting impression of arpeggios 
and chords from this bird's singing, but the song is often so rapid that 
it is difficult to separate one tone from another without intensive study. 

Melodic intervals are far more often heard in the songs of birds 
than are harmonic intervals. The Black-capped Chickadee sings two 
tones in the musical relationship of a minor third, a major second, or 
a minor second interval, sung in descending order. The Carolina 
Chickadee sings three or four tones with the above-named intervals 
and also several others occurring in the course of its wider repertoire. 
The Black-capped Chickadee's two-tone songs are so short that it does 
not seem important to know their musical key or tonality. The four- 
tone songs of the Carolina Chickadee are long enough to suggest 
definite tonalities. Both major and minor intervals are found through- 
out the Carolina Chickadee's songs. A very long list could be made 






Black - capped Chickadee Songs 

Store la 



Store Ib 



j J J j j || 

Eastern Meadowlark 
Store 2 


Western Meadowlark 
Score 3 

Two Gambcl Sparrows singing consecutive rones 
of the scale 

Score 6a Store 6b 

Eastern Meadowlark 

Leghorn Cockerel 
5 Scores 



Gambel Sparrow A Gambcl Sparrow B 

singing antiphonally 
Sun 13a Store 13b 

Field Sparrow 
Store 14 

An Eastern Meadowlark alteroftung 
related themes 

Score 15 

Song Sparrow Song Sparrow Song Sparrow Song Sparrow 
A A B A 

Song Sparrow Concert 
Store 16 

Fig. 17*11. Musical scores of birds. (Taken from life by Anne Hin- 
shaiu Wing.) 

of birds that include melodic intervals in their singing; such a list 
would include Finches, Thrushes, Meadowlarks, Wrens, Warblers, 
and many other birds. 

Melody results when a succession of tones of a given tonality are 
rhythmically sounded to express a musical idea. The effect upon the 
human listener varies with the melody and with the manner in which 
it is sung. A melodious phrase creates a certain impression in the mind 
of the hearer. If the tones belong to a major diatonic scale, a positive 
or joyous feeling is engendered in the mind of the human listener; but 


if the tones belong to a minor diatonic scale, a negative or mournful 
feeling is engendered, for reasons unknown. Perhaps birds may also 
feel the same influences. Pentatonic melodies vary in effect somewhat 
according to the order of succession of the longer and shorter 

Musical Scales. The octave is an interval that is easily under- 
stood and recognized. Within the octave, five tones, each musically 
related to the starting tone, have been recognized in human music 
from ancient times. Chinese, Scottish, North American Indian, and 
other music, including folksong of many nations, all are built upon 
pentatonic scales. More highly civilized peoples have added two inter- 
mediate tones, forming the seven-toned diatonic scales. Some other 
groups use additional tones (termed nncrownes), the resultant music 
sounding strange to European and American ears, but fundamentally 
the music of all groups appears based on the natural scales of five or 
seven tones. 

The use of tones and intervals varies with birds according to the 
species, although it is far from satisfactory to state once and for all 
that a given species sings only in such and such a manner, for birds 
are often surprising. The Prairie Warbler sings chromatic intervals 
and perhaps also microtones. The Hermit Thrush and Western 
Meadowlark seem to sing in the five-toned scale, with some excep- 
tions. The Purple Finch, Gambel's Sparrow, White-crowned Spar- 
row, Song Sparrow and Eastern Meadowlark commonly use the 
diatonic scales, major and minor, seven tones within the octave. 

It is difficult to state that the birds always use what we call pure or 
just intonation, but it is safe to state that they do not ordinarily use 
the tempered scale of fixed intonation by which our pianos are tuned. 
If a bird's tones and intervals are far wrong, probably the songs are 
classifiable as atonal, or poetic song. If basically true, a note that seems 
too high or too low may be classed as a microtone if it seems inten- 
tional, or as slightly different in intonation from human music. Per- 
haps birds, like humans, occasionally sing off-pitch. Southern 
Meadowlarks tend to sing off-pitch at the very beginning and very 
end of the breeding season, their melodies becoming clear in the 
height of the season. Song Sparrows sing less and less clearly as the 
summer wanes, until their voices are hoarse, their songs short, and 
musical tones seem almost absent. White-throated Sparrows some- 
times flat toward the end of their songs, the tendency being perhaps 
more noticeable in late summer. Western Meadowlarks exhibit the 
rhythm and general pattern of the species song in early spring, but 
tone relationships are then unrecognizable. Little variation in intona- 
tion is heard from the Black-capped Chickadees at the various seasons 


in which they sing. Birds with melodious songs with true tone rela- 
tionships are reported from many different parts of the world. Ex- 
amples are shown of the singing of two North American species, the 
Carolina Chickadee and the Lincoln Sparrow, in Score 4# and 4 
(Fig. 17-11). 

The rhythmic arrangement of bird songs varies from simple to 
complicated. Ornithologists usually time in seconds the length^of the 
song and of its parts. Musicians consider the main and subordinate 
accents in music, giving the musical "time" of the song. Gambel 
Sparrow's music often has four principal beats, suggesting four-four 
or common time. The music of the Southern Meadowlark varies, but 
some songs can be assigned waltz time, or three-four time. Some bird 
song tends to increase or to decrease in velocity, and one can use verbal 
indications to describe the change. Usually considerable musical 
training is needed in order to make the type of description used by 
musicians. However, musical indications are widely understood, and 
most bird songs are suited to this type of description. As in musical 
notations of North American Indian music, a change of rhythm may 
be indicated where it occurs. 

Responsive Singing. Birds of the same species often respond 
musically to one another, continuing or varying a musical idea. Birds 
of different species sometimes influence one another's singing, so that 
the same tones and intervals may be heard in the singing of more than 
one species in an area. An Eastern Towhee at Ann Arbor, Michigan, 
not only used some of the tones of a nearby Song Sparrow but also 
changed its own type of song to correspond to some extent with the 
Song Sparrow's song. Birds are often aware of the whistled tones of 
human beings, and sometimes show this awareness by responding in 
kind or by stopping their song. Black-capped Chickadees and Mock- 
ingbirds show this type of awareness. 

Rhythmic duets are sometimes sung by two birds of a single 
species. Male and female Chachalacas combine their songs into a 
unified whole. Black-capped and Carolina Chickadees sometimes do 
this also. The antiphonal singing of two birds may be linked in precise 
rhythm or it may not. In any event, the relationship of the melodies 
in the alternation of melodious songs is evident. 

Bird songs are usually short and unfinished by human standards. 
The Eastern Wood Pewee sometimes achieves a sense of finality by 
so ordering the succession and selection of phrases that a cadence is 
effected. The bird, however, fails to stop there, and the process takes 
place over and over. This is a phenomenon of the twilight singing of 
this species. Two Gambel Sparrows may alternate songs rhythmic- 
ally, the one song being a variation upon or a continuation of the 


other, some degree of finality being achieved in some instances. 
Again, however, the birds do this over and over. A Song Sparrow 
tends to repeat a song a number of times, then vary it so that, whether 
or not the bird is aware of the fact, some sense of musical cadence is 
achieved. Eastern Meadowlarks may alternate related themes. Hermit 
Thrushes have a limited number of themes but repeat them over and 
over in so many different orders of succession that finality appears, 
becomes lost, and reappears, no real cadence having been reached. 
Birds appear able to vary their tone successions so as to satisfy some 
feeling for musical continuity and cadence, following intuitively the 
natural laws of music used in a higher degree by human musicians. 

Melodic variation is widespread among birds, some species ap- 
pearing to extemporize. The Eastern Meadowlark and the Song 
Sparrow have many musical ideas and seem capable of inventing others 
at will. One song seems to suggest the next, and a succession of songs 
sung at a single sitting often seems musically related. An interruption 
or a period of silence may be followed by musical phrases in another 
mood. Each species sings in its own characteristic way most of the 
time, so that regardless of the melody of the moment, the accustomed 
listener can recognize the species by the song. Sometimes some char- 
acteristic of voice quality is the key to recognition when there is 
doubt, or it may be a characteristic rhythm. Occasionally the same 
melody occurs in different species. On the other hand, individual 
Song Sparrows in nearby territories at Brady's Bend, Pennsylvania, 
sang certain individual songs in so characteristic a manner that they 
could be distinguished by their melodies. In addition, these territory 
defenders sang other and less characteristic songs. 

In spite of melodic variation in the songs of a species, the kind of 
music sung is typical of the species as a whole and may even be typical 
of related species. Among species of Chickadees there may be found 
a relationship in melody, some songs of the Carolina being similar to 
some songs of the Mountain Chickadee, whereas others suggest those 
of the Black-capped Chickadee. Similar musical relationships may be 
found among the members of the Thrush family. Both tone succes- 
sions and song patterns may be to some extent inherited, the ability 
to vary the song differing according to the species. 

Birds often seem to take their cues from other singers about them. 
As mentioned previously, they frequently employ the same musical 
tones in some parts of their songs. Sometimes they may even alter 
their typical manner of singing in imitation of the song of another 
species. People who depend entirely upon quality for distinguishing 
bird song could never notice this. Sometimes several birds of different 
species may be heard singing in the same musical mode or key. A 


Song Sparrow near Ann Arbor, Michigan, for example, sang several 
repetitions of a song related to that of the Red-eyed Towhee (Fig. 
17-12). The principal difference serving to distinguish it musically 
from the singing of the Towhee consisted of the three staccato notes 
on one pitch that began the song. No sooner had the Song Sparrow 
ceased singing this song than the Towhee, rather hesitantly at first, 
began to sing. The Towhee began on C-sharp, as had the Song Spar- 
row, but with one note only, then descended to the C-sharp an octave 
lower, as had the Song Sparrow, then rose to F-natural instead of 
F-sharp, as if uncertain of what had been sung, then followed with 


Song Sparrow Red-eyed Towhee 

Fig. 17*12. A Song Sparrow sings a song similar to that of a Red-eyed 
Towhee. Then the Towhee starts to sing its oivn song, although it had 
been silent before. (Taken from life by Anne Hmshaw Wing.) 

A-sharp, forgetting, one might suppose, that the Song Sparrow had 
sung C-sharp, as the Towhee might well have done. Earlier in the 
season, a Towhee in the same area sang in seeming imitation of a 
Song Sparrow, the two songs interfering, and the Towhee going out 
of its way to change the form of its song to harmonize with the song 
that the Song Sparrow sang at the moment. Many other birds may be 
heard from time to time imitating songs of other birds of different 
species or blending their songs with them, especially in the matter of 

To review the foregoing pages, many bird songs suggest atonal 
syllables rather than melodies; some bird songs are commonly melo- 
dious only in part; and some birds usually sing melodiously through- 
out the length of their songs. There is much yet to be known con- 
cerning the musical behavior of birds. Much of the most remarkable 
music comes from some of the members of the Finch family (Fringil- 
lidae), the family of the Meadowlarks (Icteridae), and the Thrush 
family (Turdidae). These and other fine singers combine musical 
tones into brief melodious phrases comparable to the melodious 
phrases of similar length sung or played by human musicians. Al- 
though the bird music considered here is of North American origin, 
other writers have described as musical the bird song of other con- 



CHENEY, SIMEON PEASE, Wood Notes Wild: Notations of Bird Music. Boston: Lee & 

Shepard, 1891. 

CRAIG, WALLACE, The Twilight Song of the Wood Pewee Myiochanes virens Lin- 
naeus: A Study of Bird Music, New York State Museum Bulletin No. 334, 1943. 
INGRAHAM, SYDNEY E., "Instinctive Music," Auk, 55(1938):614-628. 
MATHEWS, F. SCHUYLER, Field Book of Wild Birds and Their Music. New York: 

G. P. Putnam Sons, 1921. 
NICHOLSON, F.. M., and LUDWIG KOCH, Songs of Wild Birds. London: H. F. & G. 

Witherby, Ltd., 1936. 
OLDYS, H. W., "Parallel Growth of Bird and Human Music," Harpers Magazine, 

105 ( 1902 ):474-478. 
RUPPEL, W., "Physiologic und Akustik der Vogclstimme," Journal fur Ontithologie, 


SAUNDERS, ARETAS A., Bird Song. New York State Museum, Handbook No. 7, 1929. 
SAUNDERS, A RE FAS A., A Guide to Bird Songs. Garden City, N. Y.: Ooublcday & 

Co., Inc., 1951. 
WITCHELL, CHARLES A., The Evolution of Bird-Song. London: Adam & Charles 

Black, 1896. 


Courtship and 
Nesting Habits 

In the cold light and the figurative glare of biology, a bird may be 
said to live but for one purpose: reproduction of its kind that the 
thread of the species may remain unbroken. In reality, to accomplish 
this seemingly brief task takes much of a bird's effort. Because only 
a part of the year is suitable breeding time in most areas except for a 
few tropical ones, in a very real sense a bird survives during the non- 
breeding season in order to function as a potential parent of the breed- 
ing season. Many other animals do likewise, but some forms of life 
can suspend operations during the unfavorable season, so to speak, by 
such means as encysting or calling "time out" as in aestivation or 

The reproduction habits of higher animals in general follow com- 
plex patterns and involve complicated behavior. These all seem to be 
functional. Because so many people are interested in birds, their 
breeding habits have been studied more than have those of any other 
class. The courtship behavior of birds includes remarkable perform- 
ances. They have been shown repeatedly to embrace "formalized" 
rituals and ceremonies. 


Function of Courtship. In the simplest analysis, courtship serves 
chiefly as the combination or magic "sesame" unlocking the door of 
reproduction. Only the right turns of the figurative biological dials 
permit the pairing of birds. In so doing, courtship provides much of 
the reproductive isolation in the bird world. "Psychological isola- 



tion" during the breeding season may operate through differences in 
plumage, voice, or habits (Skutch, 1951). The intricate courtship 
pattern of one species thus brings response and pairing with indi- 
viduals of that species only. Without some channeling of the repro- 
ductive interests of the many species, each to its own kind, biological 
chaos would surely result. 

Because birds cannot communicate and receive identification and 
scxualsignals by scent (a rather accurate indicator of the reproductive 
and sometimes physiological state of a mammal), the complicated 
courtship behavior of birds may be a more involved way of accom- 
plishing the same thing. By means of visual and vocal signals, the 
sexes may (a) clearly and recognizably indicate the desire for pair- 
ing, (b) stimulate each other, or (c) reach a satisfactory state of 
accord. A male already in the prime of the seasonal sexual cycle 
might very well pass beyond the efficient stage for carrying on his 
share of the reproductive burden if mated to a delayed female. To 
illustrate this, one might imagine a resident male Robin in Oklahoma 
pairing off in the spring or wasting his early reproductive possibilities 
on a female Robin migrating to the Yukon and not ready to breed 
for many weeks to come. Clearly, it is an advantage to the species 
for its members to signal to each other their state in the breeding cycle, 
automatically or upon demand. 

Recognition of Sex. The sex of birds having marked sexual 
dichromatism (or other sexual dimorphism) often may be recognized 
by appearance (see Chapter 8). The presence of the "moustache" of 
the Flicker (page 131 and Fig. 7-17) identifies its wearer as a male 
to males and females alike (Noble, 1936). Probably birds recognize 
the opposite sex instinctively. A young Mallard female, newly adult 
and previously inexperienced with the male, having been reared by 
the mother alone, recognizes the green-headed male as such. Most 
birds seem able to recognize the cues or signals of sex, whether vocal, 
visual, or behavioral, with little if any trial and error period. Yet it 
must be recognized that learned habits (imprinting) may influence 
many things in bird life. Most if not all birds reared under alien 
circumstances if free to do so would return to the ways of their kind, 
though the road might prove a rocky one. The young Cowbird in the 
summer or fall deserts the foster species to consort with its kind; a 
female Cowbird is able ifF recognize the male, though reared alone in 
a foster home. The s^jjKunay be said of birds reared by hand; yet one 
should interpret with cautihA happenings in the lives of zoo birds and 
others exposed to man's confinement. 

Species having the sexes alike, and many others, evidently distin- 
guish sex largely by action, either alone or in combination with Jbunds, 


A male and female Song Sparrow meeting for the first time in the 
spring and often after loftg acquaintance react in the manner cus- 
tomary to the sex. A male bristles up to a stranger, which, if it is 
another male, reacts aggressively in kind or flees; but if it is a female, 
the stranger will probably crouch submissively or in an inviting man- 
ner, often with a peculiar call, or flee (Nice, 1939). But the English 
Robin has to learn the hard way, it seems, for the female may be the 
more aggressive. Sex recognition appears by no means to be instan- 
taneous; the female searching for a mate persists in returning to the 
territory, whereas other females move off (Lack, 1946). The Yellow- 
eyed Penguin, however, always knows the sex of another bird (ju- 
venile or adult) on sight, though the sexes look alike. The observer 
himself learned to distinguish females by their "sheepish look," and 
presumably the Penguins themselves could do as well (Richdale, 

The whole subject is a complex one. Birds may interpret (hence 
recognize) various signals to mean various things at various times, not 
necessarily of a sex-recognition nature, and methods of sex recogni- 
tion are still largely unknown to us. In any event, birds may clearly 
get to know each other as individuals (page 383 and Fig. 19-5). Bob- 
whites can detect strangers in the covey. Common Terns will recog- 
nize their mates in the air; mated Pintails identify each other at 300 
yards from the nest and English Robins at 30 yards. The Smooth- 
billed Ani distinguishes individuals. Song Sparrows know each other, 
Black-headed Gulls know their associates, and the Crimson-crowned 
Bishop Birds know their neighbors (Armstrong, 1947, 1950). 

Mating Habits. Animals follow several patterns of mating habits, 
which have been termed polyandry, polygamy, promiscuity, and 
monogamy. Monogamy is probably the most usual rule of mating in 
the bird world. At the minimum, the duration of the mating bond 
in monogamous pairings may be short, almost momentary and for 
insemination only; for all practical purposes, this is really nonpairing. 
In some species, the pairs may remain mated for some time, usually 
until incubation begins. In more constant ones, a pair may remain 
mated through one brood, remating often with each other for subse- 
quent broods. In many common Passerine birds, the pairs remain 
mated through the season, sometimes raising several broods during the 
period of the mating bond. Some birds, (e.g., Geese, Swans, and 
larger predators) may form a mating bond for life. Monogamy may 
be seen to result in pairing off in unions of varying length. 

Polyandry occurs rarely among birds. It may indicate a probable 
surplus of males, though reliable figures are difficult to find. The 
Button-quails (Turnix) and Painted Snipe (Rostratula) practice poly- 


andry (Mayr, 1939). It occurs at times among species in which the 
female takes the more aggressive role in courtship behavior, as among 
the Phalaropes. A pseudo polyandry in which several bachelor males 
attach themselves to mated pairs has been reported in Central Ameri- 
can Bush Tits. Attachment of a bachelor Bob-white to a mated pair 
has been reported. 

Polygamy is the form of polygyny having one male mated with 
two or more females. For it to function successfully as a species char- 
acteristic, as distinguished from sporadic or irregular occurrence, the 
males apparently must outnumber the females. This has been noted 
in several species, either by means of differential mortality or de- 
ferred maturity in the male. Species practicing polygamy usually 
have the family duties nest building, incubating, caring for the 
young done by the female alone. Grouse tend to be polygamous, as 
are the Turkey and various Pheasants. The Red-winged Blackbird 
among Passerine birds may practice polygamy. In the case of the Red- 
winged Blackbird and Ring-necked Pheasant, the several females 
establish subtcrritories within the territory of the male, which sub- 
territories they hold against each other (page 226). 

Sporadic polygamy has been reported in many birds, some of them 
otherwise rather rigidly monogamous, others less rigidly so. Among 
monogamous birds known to have had polygamous examples are such 
birds as the House Wren, American Robin, American Marsh Harrier, 
Song Sparrow, and White-crowned Sparrow. Sporadic polygamy 
among the Passerine birds probably occurs with greatest frequency 
among the Weaver Finches. 

Whether or not true promiscuity occurs among birds as a rule of 
mating habits rather than as aberrant behavior has not been determined 
clearly. It has been reported for the Ruff. It is presumed to occur 
among Grouse whose males gather at mating grounds to which the 
females resort for insemination. The Cowbird among Passerine birds 
is reported to be promiscuous, but its habits only give that appearance 
because the Cowbird lacks the ordinary nesting behavior (Laskey, 

Courtship Patterns. Three general patterns of courtship can be 
recognized, though there are numerous variations and overlappings. 
The patterns may be listed as CQjjinninA^J^m^.etltive or nonterri- 
torial, aQ&tetritori&L It may well be that further knowJedgcToT the 
individual courtship patterns of the many thousands of species un- 
studied will revise considerably our knowledge of these matters, just 
as it no doubt will with many other phases or bird life. 

Prairie Chickens, Sage Grouse, Sharp-tailed Grouse, and Black 
Grouse illustrate the workings of communal courtship, the first of 


the three patterns thus distinguished. The males gather at favored 
courtship grounds (called variously by such terms as leks, arenas, 
booming grounds, and dancing grounds) to which the females come. 
A large number of birds pair off while still in flocks, while on the 
winter grounds, when in migration, or when in the breeding range, 
such as among many Waterfowl and other water birds. Because this 
appears in public, so to speak, and thereby invites competition .and 
interference from others, it may be termed competitive or nonterri- 
torial for convenience. The majority of common birds, however, 
take up a territory before pairing off. Territorial courtship is there- 
fore rather private, and the territory dominates its occurrence. Terri- 
torial courtship implies initiative in the female as measured by her 
searching for territory or for a male fixed in his territory. 

Mate Selection. We have little actual knowledge of what deter- 
mines or signals the actual mate selection as distinguished from the 
courtship performance itself. Much has been written about selection 
as a form of evolution, but little of selection as a form of choice in bird 
life. In many territorial birds, selection or choice of a mate seems to 
be something of a haphazard or chance affair. Birds tend to return 
to their previous territories, the places familiar to them, so that 
chance is subject to this trait. A female searching for a mate may 
exercise some choice of selection, but it may be that she pairs off with 
whatever male she is with at the crucial moment of her sexual state. 
It may be that much of the choice may actually be selection by mu- 
tual tolerance or perhaps even by default. A male or female using the 
same area may mate by the simple process of accepting the situation 
without any serious "screening" other than for being of the same spe- 
cies and in the appropriate sexual condition. 

But when two or more birds compete for one of the opposite sex, 
the matter is clearly not so simple as this would indicate. A common 
Tern indicates acceptance of a mate by exchange of a fish or other 
offering. A female Mallard indicates her selection when flying by al- 
lowing one male of several contenders to fly alongside her and per- 
haps by touching him with her bill. In the water, the male bobs his 
head and an accepting female bobs back. But just how the majority of 
birds signal acceptance of each other as mates is little known. It may 
be that the acceptance is really signaled by mutual use of the behavior 
patterns that mated pairs use with each other which might indicate 
that if the birds do use them, they are mated, if not, they are not 

Courtship Performances. In a sense, courtship is a series of dis- 
play performances, often with calls and songs, though sometimes per- 





formed only by posturing and displaying of the plumage (Chapter 7). 
The number and variety of display performances seem limited only 
by the number of bird species. Sometimes those of the male and 
female are very much alike (Fig. 18-1). Because each species has 
several to many meaningful poses, perhaps destined to release a re- 
sponse (Tinbergen, 1948), the total number in the bird world is al- 
most astronomical. Numbers of them have already been described 
(e.g., Armstrong, 1947). Many birds, for example, put on "courtship 

Fig. 18*2. Perches used and flight paths followed between calls by a 
wale Violet-eared Hinmmngbird. (After Hehmtth O. Wagner, "Notes 
on the Life History of the Mexican Violet-ear" Wilson Bulletin, 57 

flights" involving both male and female, the former usually the pur- 
suer, the latter the pursued. Often special songs or calls are part of 
the flight. Some involve display of color patches, like the white throat 
and white wing patches especially accentuated by the Nighthawk. 
The flight gait itself may involve differences peculiar to nuptial flight. 
Courtship flights in some birds may be by the male alone. The 
flight song of the Vermilion Flycatcher (see Fig. 17-8) is in a sense 
a courtship flight. (It should be recognized, however, that distinction 
between courtship and territorial behavior has not been clearly estab- 
lished.) A variation in which the bird calls from a perch but changes 
perches between calls (averaging 2 minutes and 40 seconds apart) 
occurs in the Violet-eared Hummingbird (Fig. 18-2). Many birds 
of the open prairie or marsh have courtship flights involving both 
male and female (which if mated may more properly be considered as 
nuptial flights) or the male alone. That of the American Marsh Har- 
rier is particularly well known. Even some birds of the forest, like the 



Red-breasted Nuthatch, Woodcock, and Ovenbird have courtship 

The courtship postures and displays of some game birds have been 
studied in considerable detail. Fig. 18-3 shows several poses of the 
Ring-necked Pheasant, male and female. But no less elaborate in 
many cases are the performances of other birds, some of which in- 
volve complex dances associated with equally complex actions. There 


Fig. 18-3. Postures of wale or female Ring-necked Pheasants, (a) In- 
tinridation display of dominant male to submissive male, (b) courtship 
display of wale to female, (c) pose of nonterritorial male 'while trespassing 
in the territory of another, (d) "flirting hop" posturing of a female, (e) 
stretch posture of female to male, (f) half -squat posture of female, (g) 
posture of hen without chicks, neck not stretched, (h) posture of hen 'with 
chicks, neck stretched. (After Richard D. Taber, "Observations on the 
Breeding Behavior of the Ring-necked Pheasant" Condor, 5 1(1949):! 63.) 

is a conspicuous correlation between the movements of a displaying 
bird and its special charms of form or coloration. The Lady Amherst 
Pheasant erects the neck-ruff on the side turned toward the female. 
A Ruby-crowned Kinglet (see Fig. 7-18) erects his crown, the Um- 
brella Bird exhibits his bright-red air pouches, and the Snow Bunting 
displays his back pattern to the female (Fig. 18 -4). 

One of the most striking of performances is the distinctive upside- 
down display of several Birds-of-paradise, all famed and resplendent 
exhibitionists. Male Bowerbirds, near relatives of the Birds-of-para- 


Fig. 18-4. Back display of the Snow Bunting. The male displays his 
variegated black and 'white pattern, and in so doing, 'walks slowly away 
from the female, his back toward her. (A^er N. Tinbergen, "Social 
Releasers and the Experimental Method Required for Their Study," 
Wilson Bulletin, 60(1948):6-5L) 

disc (also of New Guinea and nearby islands and Australia), have 
the unique habit of building tiny ceremonial houses (bowers) and 
walks furnished with display objects (usually blue in the wSatin Bower- 
bird's bower, which species also has intensely blue eyes) (Marshall, 
1950). The whole process of the male's display before the bower ap- 
pears to be substitution of an inanimate object for the primary sexual 
attraction itself and may serve in maintaining sexual stimulation 
despite establishment of territory early and at a time highly disadvan- 
tageous for reproduction. To add to the several remarkable features 
of the performance, some males make a "paint" or charcoal of fruit 
pulp mixed with oral secretions and apply it to the inside of the bower 
by means of a soft, fiber swab. This has been suggested as having 
some remote connection with courtship feeding, perhaps as a substi- 
tute offering to a substitute object. 

Incubation and Courtship Feeding. Males of many species feed 
the incubating female, perhaps the best-known example being that 
found among the Hornbills. The female of the Hornbills incubates 
while the male feeds her; she has been imprisoned by the male who 
closed the hole except for a port through which he passes food. 

Courtship feeding, as distinguished from incubation feeding, has 
been reported among birds in more than forty families of sixteen 
orders. Among common birds, courtship feeding seems unreported 
in Loons, Grebes, Petrels, Grouse, Auks, Woodpeckers, Thrashers, 


Starlings, Vircos, and Weaver-finches. In others, courtship feeding 
varies from a regular trait to a rare event (Lack, 1940). 

In the act, the female usually adopts an attitude and uses calls al- 
most identical with those of a young bird begging for food. The 
male may put the food into her mouth. Some birds regurgitate food 
for the female; the male Herring Gull deposits it on the ground, and 
the male Pigeon feeds from his crop. The Common Tern is especially 
known for using a fish in courtship feeding. Even the common barn- 
yard rooster calls the hens to food. In the Button-quails, the female is 
reported to feed the male. Substitutes for food may be offered. A 
Tern may present a stick instead of a fish. The Adelie Penguin 
offers snow, though this seems to be done during incubation and may 
be related to incubation feeding. The Gannets merely touch bills 
together, which may be the minimum act in the performance. "Bill- 
ing and cooing" by some birds may be a substitute for courtship feed- 
ing, just as touching the bill in the Gannets may be the minimum of 
the substitute. 

Because courtship feeding seems not primarily concerned with 
food as nutrition (which incubation feeding seems usually to be), it 
may be a vestigial act now part of display and courtship. It has been 
suggested that the infantile parent-child relationship in this way may 
be symbolized in adulthood. Its primary function may be to main- 
tain the mating bond, for it is found mainly among those species in 
which both sexes care for the young. Of this, however, there are 
marked exceptions. In some species, courtship feeding seems a neces- 
sary prelude or signal for copulation. 

Copulation. In its immediate purpose, courtship functions to 
establish a harmony between the sexes to accomplish insemination of 
the female at the proper time in the reproductive sequence. The 
dominance of the male over the female (sex dominance) also plays its 

Fig. 18*5. Signals or stimuli may precede copulation as in the "begging 
movements" of the Herring Gull (left) or the "preening movements" of 
the Avocets (right). (After N. Tinbergen, "Social Releasers and the Ex- 
perimental Method Required for Their Study, Wilson Bulletin, 60(1948): 


part in providing a submissive receiver. But only when birds have 
been stimulated sufficiently and at the proper time will copulation 
take place. The sight of birds copulating may induce imitation or 
suggestion in others, but it may also elicit attack on the copulating 

In some species, copulation occurs upon invitation of the female 
or in response to a vocal signal. It may occur with little preliminary 
ceremony, the female merely squatting. But in most birds, the male 
seems to initiate the preliminaries or they may be largely mutual. 
Usually this involves such actions as body contact, billing and cooing, 
nodding and bowing, flight pursuit, or even preening (Fig. 18-5). 
The preliminaries may be violent buffeting or seemingly violent at- 
tacks by the male upon the female; but violent though this may be, 
it appears to be a regular part of the ritual in some species. 

The amount of semen produced in the Domestic Rooster varies 
from 0.05 cc. to 1.00 cc. per completed copulation. The sperm 
count averages nearly a billion for each discharge (Parker et al., 1942) ; 
the number of spermatozoa ranges from none to ten million per cubic 
millimeter and averages about three million, a count substantially 
greater than that in domestic mammals. The spermatozoon measures 
about 90 microns in length, three-quarters of which is tail, a part lost 
soon after reaching the oviduct (see Fig. 6-1). 

Generally the period during which copulation occurs lasts but a 
short time, and copulation usually ceases when the first egg is laid. In 
some species, copulation may occur a month or more before egg lay- 
ing, more than 2 months previously in the Gentoo Penguin (Richdale, 
1951). Yet copulation in autumn and even on the winter grounds 
3,000 miles from the breeding range has been reported for the Euro- 
pean Roller (Armstrong, 1947). Fertilized eggs seem to be laid not 
earlier than the second day after copulation. Experimentally, the per- 
centage of fertile eggs from domestic hens declines rapidly in a week 
or so after the last copulation. But Sage Grouse hens lay no eggs for 
two or three weeks after copulation, which seems to occur but once 
annually (Scott, 1942). 


Breeding Seasons. In but few species of the equatorial regions 
have ornithologists found much evidence for truly nonseasonal breed- 
ing (Baker, in DeBeer, 1938). In higher latitudes, breeding is annual 
in the course of the seasons. As someone has said, it follows the sun. 
But in the Tropics and Subtropics, the breeding season has a complex 
character; yet seasonal events in nature are not entirely absent. In 



the wet Tropics, nevertheless, no regularly recurring period is avail- 
able to which breeding season control may be related (Baker, in 
DeBeer, 1938), and ornithologists sometimes tend to postulate some 
rhythmic, internal control. 

The egg season of eight species of Coots (Subfamily Fulicinac) 
with wide latitudinal distribution illustrates the tendency for some 
species to show two breeding seasons in the Tropics. Each season, 

80 N 

Fig. 18-6. The egg-laying season of eight widely distributed Coots 
(Subfamily Fulicinae). Each hemisphere has its own breeding season that 
carries across into the opposite one. In this type of distribution, nesting 
begins earliest in mid-latitudes and later toward the poles and Tropics. The 
diagonally shaded area represents the usual season and the cross-hatched 
area the exceptional season or the usual one of a few species. (After Baker, 
by permission from Evolution Essays on Aspects of Evolutionary Biology, 
ed. by G. R. DeBeer, p. 165. Copyright, 1938, The Clarendon Press, 

in fact, is that of the respective Northern or Southern Hemisphere 
carried into the Tropics and even across the Equator (Fig. 18-6). 
The birds breed earliest in the middle latitudes and later toward the 
poles and Tropics, though data from the Southern Hemisphere are 
not sufficient to demonstrate this clearly. In some species, however, 
the egg season becomes progressively later from the tropical region 
poleward in both hemispheres, that in the Southern being half a year 
behind that of the Northern Hemisphere. 

But some species seem insensitive to latitude and breed at about the 
same time throughout their ranges. The Secretary Bird, for example, 


breeds in July and August in the Sudan north of the Equator, as well 
as in tropical South Africa. 

The Sooty Tern usually breeds in the appropriate spring or early 
summer season north or south of the Equator, with a tendency to 
breed earlier nearer to the Equator. Yet local controls may influence 
this, as may also the apparent source of the breeding birds. Birds 
ranging perhaps in the northern oceanic region may return south to 
breed at times different from that of nearby birds from the Southern 
Hemisphere. That might explain differences in breeding time, such as 
of Terns nesting on some Hawaiian islands in both June and Novem- 
ber (Richardson and Fisher, 1950). 

A most interesting breeding-season rhythm, possibly the most un- 
usual yet reported by ornithologists, occurs among the Sooty Terns 
of Ascension Island, 8 South Latitude in mid- Atlantic. The Tern, 
known locally as Wideawake, breeds in large colonies called "fairs," 
hence "Wideawake Fair." The members of the fairs disperse over the 
tropical South Atlantic but return to Ascension Island to breed (page 
292) at intervals averaging about 9.7 months (Chapin, 1954). A 
time chart shows that the pattern measures exactly ten lunar months, 
which may be only a coincidence. No known environmental perio- 
dicity accounts for the special timing, and a physiological relaxation 
period has been suggested. In the Seychelles Islands of the western 
Indian Ocean, the Sooty Tern breeds in July and August, their arrival 
and breeding time apparently being governed by the monsoon 
(Moreau, 1950). 

The nesting dates of the Horned Owl in North America from 
Florida northward show well a latitudinal progression (F. M. Baum- 
gartner, 1938). How it may operate southward is not known. Nest- 
ing in southern Florida begins in late November, in New England in 
early February, and in Labrador in late March. This gives a north- 
ward shift of 85 to 90 miles of latitude for each week of spring. The 
breeding season starts later by 3 to 4 days for each degree of latitude 
and 100-125 meters of altitude, which accords with the Hopkins 
bioclimatic law (Johnston, 1954). 

Length of the Breeding Season. It may be considered as axio- 
matic that for each species the breeding season is long enough to raise 
the minimum number of young maintaining the population. But it 
should not be said that birds necessarily breed everywhere as rapidly 
or as slowly as they can. Bird life in the middle and higher latitudes 
of the Northern Hemisphere has a pronounced seasonal rhythm 
readily seen by even the most casual observer. Birds, for all prac- 
tical purposes, can be said to breed beyond the Tropics in the 
"spring" of the year, but one must recognize a lack of precision in 



Fig. I8-7. Canada Jay on nest at 10 F. below zero, Prince Albert, 
Saskatchewan, April L (Photograph by Fred G. Bard.) 

such a generality. A few birds, for example, begin nesting in the early 
winter or even late fall along the tropical side of the middle latitudes. 
Bald Eagles in Florida may start repairs to their eyries in the last week 
of September or in early October and may begin to lay eggs by the 
first week of November (Broley, 1947). Individual Eagles are 
usually consistent in nesting (i.e., in being either early or late), some 
nesting several months after the early ones. Individuals and colonies 
of Brown Pelicans have on occasion nested earlier each succeeding 
year to push nesting forward from February and March to October. 

Among early nesting birds are the Horned Owl and Horned Lark, 
which may nest in northern United States and southern Canada so 
early that snow may yet cover the nest. The Canada Jay regularly 
nests early, often long before snow has left the northern woods, when 
the temperature may be below zero (Fig. 18-8). By its insulation, 
the thickly constructed nest no doubt aids greatly in preserving the 
needed high incubation temperature. 

Some birds may be consistent late nesters. The American Gold- 
finch, for example, regularly waits until practically midsummer before 
nesting, long after most other birds have started and many have fin- 
ished. The Gray-crowned Rosy Finch of the snow line in the Rockies 
nests late also, as does the Evening Grosbeak. 


Many birds start early and nest more or less continuously through- 
out a long season from early spring until late fall, often bringing 
off several broods. But others nest but once or twice during a rela- 
tively fixed interval. Among the former are the Mourning Dove, 
House Sparrow, and Cardinal that nest in some parts of the United 
States from February or March until September or October. But 
many species, especially though not always in the northern regions, 
nest for a short season only. 

Seasonal Variations. Anyone who has recorded the finding of 
nests will soon be aware of differences in the time of first nests or the 
peak of nesting from year to year (page 213). An observer in New 
York state found that Black-capped Chickadees laid the first egg 
about May 5 in 1941, some two weeks earlier than in 1940, when the 
first laying was about May 18. 

It has been demonstrated that the phases of the nesting season may 
be more or less telescoped in the North as compared to farther south. 
The White-crowned Sparrow at the Canadian border uses less than 
two-thirds of the time taken by those of central California for the 
active part of the reproductive cycle (Blanchard, 1941). A compari- 
son of Prothonotary Warblers (page 364) nesting in Tennessee and 
Michigan (about 400 miles farther north), for example, shows that 
the latter birds nest later, use less time in preparatory activities, have 
larger eggs, lay larger sets, and only occasionally attempt second 
broods. The nesting season varied from 69 to 73 days in Michigan and 
from 101 to 128 days in Tennessee (Walkinshaw, 1941). 

Conditions Influencing Breeding. That the environment influ- 
ences the activities of birds hardly seems necessary to mention. The 
ways by which it may do so seem almost limitless, so many and so 
diverse are they. In general, the environmental influences fall con- 
veniently into such common classes as light, climate (or weather), and 
vegetation. * 

In the long run, suitable conditions for rearing of young, such as 
food and cover, may be the underlying determinant to which birds 
have become adjusted. Biologists have long known that light, either 
as a total amount or a daily increase (see Chapters 6 and 16), will 
bring birds into breeding condition. But the actual breeding itself may 
be retarded or suppressed when territorial or social conditions are 
unsuitable. An example of this may be found in the reservoir of birds 
unable to find territory for themselves and thereby not breeding, yet 
they may do so if territories become available (page 257). Cool 
temperatures have repeatedly been shown to delay nesting just as 
warm ones may accelerate it. But cool temperatures within the usual 


variations of the habitat seem unable to stop breeding, even though de- 
laying it. 

Birds of tropical regions may be attuned to wet and dry seasons 
irrespective of calendar months. Presumably they are governed by 
events leading up to the season (in which they breed) or perhaps the 
birds have much shortened the preliminaries that begin after the 
coming of rains. There are examples of birds that in dry tropical re- 
gions breed before the rains, though most seem to await the start of 
the,rainy season (Baker, in DeBeer, 1938). The famed Finches of 
the genus Geospiza in the Galapagos Islands breed when the rains 
begin; it is said that the sound of falling raindrops stimulates captive 
birds to sing (Lack, 1950). In wet tropical regions with well-marked 
differences in rainfall, birds tend to breed toward the end of the wet 
season. But in wet tropical regions with less well-marked differences, 
breeding tends to take place, for the most part, in the drier part of 
the year. 

Many birds of the Arctic fail to breed because of failure to find 
safe nest sites, inability to find good cover, or because of shortage of 
food (Marshall, 1952). 

Influence of Social Stimulation. The influence of imitation or 
suggestion should not go unnoticed. In close quarters, as on the nest- 
ing grounds of colonial birds, the action of one bird or pair may be 
taken up by others. Early pairing activities of one or two birds in a 
flock may set off this behavior in others. Nest building by a single 
pair of Terns seems sufficient stimulus to start a series of nest con- 
structions by others, just as a sudden flight by one bird in some flocks 
may be the impetus to cause all to fly. But it should be expected that 
these reactions would be stimulating only to favorable subjects. 
When a population drops below a minimum level, the lack of social 
stimulation may be important in biological success (Darling, 1938). 


Selection of the Nest Site. Among birds that form no lasting 
mating bond and among polygamous ones, it is usual for the female 
alone to select the nest site. In the common birds of the field, forest, 
and garden, the male selects the neighborhood, usually alone (see 
Chapter 12). But it hardly seems reasonable to suppose that a male 
selecting a territory would be unaware of potential nest sites. Pref- 
erences in sites are clearly instinctive, and it seems logical to suppose 
that a male carries such instincts in his inherited make-up, just as 
does the female. Their expression, however, may be inhibited in the 
male or stimulated in the female, for she generally takes the initiative. 


Among birds having very special nesting-site requirements, the 
male may actually select the site, often well in advance of rinding a 
mate. The shortage of holes and the competition for them among 
hole-nesting birds, for example, make successful breeding dependent 
upon the male having found one when the finding was good namely, 
early in the season. A male House Wren arrives ahead of the female 
and promptly seizes upon a nest box in the garden as the center of 
his territorial attentions. The male House Sparrow takes over some 
cranny as his own and chirps his territorial claims. As soon as a 
female appears upon the scene, he pops in and out to signal his pos- ( 
session and, it may be supposed, the suitability of the site; no doubt 
it serves also as an enticing gesture for a responsive female. 

Within the territory of the male, the female looks for suitable 
nest sites, often attended by and on occasion helped by the chirping 
and singing male. The female (and sometimes both) tries out forks, 
tangles, and other likely sites by squatting in them as though for the 
feel of the site. But on occasion, the female apparently rejects all 
sites within the male's territory and chooses one beyond. When this 
happens, the male must add the new area to his realm, often by fight- 
ing it out with another in possession. Even among those species in 
which the male selects the actual nest site, the female may turn it 
down and build in a new place. Some males build a partial nest at the 
site or place there some nesting material, which may be added to by 
the female, thrown out altogether in favor of a fresh start, or rejected 
for some other site. 

Nest Material. Birds do many things that seem marvelous to us, 
and construction of a nest surely is one of them. A newly adult fe- 
male, at her first nest-building job an inexperienced bride, so to 
speak builds a complicated nest in every way the typical nest of her 
progenitors, whether she lived in one like it or grew up in an arti- 
ficial environment. (Among some species, succeeding nests have 
been found superior to earlier ones, which indicates improvement 
with maturity and experience.) /Yet the ability of the bird to weave, 
entirely by instinct, challenges the imagination j(Fig. 18-8). 

Birds generally use the handiest material, but selection character- 
izes the species and to some extent the larger taxonomic groups. 
Birds of a genus and family, for example, tend to build rather similar 
nests. But there are many exceptions and variations, especially among 
birds of wide adaptive radiation, like the Icteridae.jjr Common birds 
use mosses, lichens, grasses, fibers, stems, and small twigs. Substitu- 
tion of man-made or man-associated materials may occuip (see Table 
10*2). The American Goldfinch that uses plant fluff will build a 
nest of cotton or sheep's wool. Wrens have used wire clippings in 



place of the usual sticks. American Robins often use string, yarn, or 
twine. Chipping Sparrows had no horsehair available before the 
coming of white man in America, at least not since the Pleistocene. 
Horsehair seems to be but a substitute for fine plant fibers. 

Special nesting material, however, may be the vogue for some 
species. The American Robin builds a nest reinforced with mud and 
fiber. A Barn Swallow builds a mud nest on the beams of a barn as 
a substitute for a protecting cliff wall, broken tree, or hanging ledge. 

Fig. 18-8. Weaving by the Red-billed Weaver-finch. Left. The bill 
serves as the shuttle, the feet holding the weaving strip to the limb. Lower 
right. Simplest and most commonly used stitch. Upper right. A stitch 
used in conjunction with the lower one. Arrows indicate direction of 
weaving. (After Herbert Friedwann, "The Weaving of the Red-billed 
Weaver Bird in Captivity" Zoologica, 2 (1 922) :3 5 5 -31 2.) 

The Cliff Swallow, however, goes a step further; it builds a retort of 
mud stuck to a cliff. Yet it will use the eaves of a barn or the concrete 
of a bridge. The "glue" that holds the mud together comes from the 
oral secretions, as it does also in the Swifts (page 61). 

Few birds go far for nest material, a few score yards at most for 
the average dooryard bird and seldom more than a few dozen rods 
for any bird except possibly those needing special material. Eagles 
may bring articles long distances, but usually only when the nest is 

A great many individual pieces make up the nest of a Magpie or 
Bald Eagle. The former may be as big as a wash tub, the latter a dozen 
feet across. A nest of the Alta Mira Oriole from Tamaulipas, Mexico, 
measured 6 by 8 inches in diameter and 20 long; it contained by count 
3,387 separate pieces of grass and plant fiber, many from 36 to 50 
inches long, the total weighing 131.1 grams, air dry. A Rose-throated 
Becard nest from the same state measured 10 by 11 by 19 inches in 
size and contained 1,844 separate pieces of leaves, twigs, grasses, and 


fibers, several from 18 to 41 inches long, the total weighing 342.7 

9'ams, air dry. A Purple Finch nest from the Adirondacks of New 
ork measured 6 by 5 by 5 inches and contained 753 separate pieces, 
largely grass, several from 10 to 15 inches long, the total weighing 
53.2 grams, air dry.* The Becard and Finch measure less than 6/ 2 
inches in length, the Oriole but 9. One must surely credit such birds 
with expenditures of relatively prodigious amounts of energy. If 
one were to hazard a guess, perhaps 400 to 800 pieces would be aver- 
age for the number of items used in building a Passerine nest. 

Nest Construction. It has been said that in general if the male 
differs strikingly from the female, especially if he is brightly colored, 
or if he is an ardent and adept musician, he takes little part in the nest 
building. But when males and females are alike, they may work to- 
gether on the job, or the/fnale may bring nest material but the female 
does the actual fitting or it into the nest structure^Among common 
birds, the male helps some either by carrying nesting materials as in 
the Cardinal, or by helping in the actual construction as in the Crow. 
All possible variations seem to be found. The male of some, perhaps 
most, Weaver-finches, for example, usually does all the building; but 
among the Orioles, the female builds the nest. Among hole-nesters 
that excavate their own cavities, both sexes generally work together. 
The male and female Woodpeckers and Kingfishers also work to- 
gether on the hole as well as the nest chamber. Both sexes of Cliff, 
Barn, and Bank Swallows participate in nest buildingy 

Once started, nest construction may be very rapid, though like 
other things in nature, variations occur. Usually birds start nest 
building shortly after forming the mating bond; it appears to be 
delayed in some, like the American Goldfinch. Sometimes Wood- 
peckers may excavate a cavity the fall before and use it for winter 
roosting quarters and often for nesting later on. First or early season 
nests usually take longer than later ones. Ten Prothonotary Warbler 
nests constructed in May required 7, 6, 11, 8, 2, 3, 2, 4, 6, and 4 days 
respectively to build; nine constructed in June required 6, 1, 1, 1, 4, 
1, 1, 1, and 1 days (Walkinshaw, 1938).\ Birds of the short seasons 
of higher latitudes take less time than birds of related species or of the 
same species farther toward the Equator (page 360) .y The Alta Mira 
Oriole may use 3 to 4 weeks in building a nest in Tamaulipas, but the 
Baltimore Oriole farther north builds in 10 to 12 days. 

The Red-billed Weaver, when given a choice of color, showed 
preference in this order: red, orange, yellow, green, blue, violet, and 
blaoK. The last three named, however, may not have been used in 
significant amounts (Friedmann, 1922). 
r Original data. 








Nest Location. Each species has an instinctive pattern of choice 
for a nest site (Fig. 18-9). The Cardinal seeks vines, bushes, thickets, 
and similar places for nesting in all parts of the range. A Mourning 
Dove chooses a tree limb or a clump of twigs that provides a suitable 
platform. It may nest on the ground. Most New World Warblers 
prefer forks among the branches, but the Prothonotary chooses a 
cavity, the Kirtland the ground, and the Yellow a bush. Cormorants 
nest on rocks of islands and headlands, and some colonies may take 
to the trees (Fig. 18-10). Herons and Egrets nest in trees and some- 
times in bushes of off-shore islands along the Gulf of Mexico. 

Throughout many groups of birds runs the habit of nesting in tree 
cavities (but this does not indicate relationship). Ducks, Hawks, 
Parrots, Owls, Flycatchers, Swallows, New World Warblers, Chick- 
adees, Nuthatches, Wrens, Thrushes, and Weaver-finches have spe- 
cies that nest in holes. Few of these are able to make holes of their 
own, which in a sense seems to be the mark of "true" hole-nesters 
like the Woodpeckers. 

The height above the ground varies, but each species tends to fol- 
low a pattern. Some may be high nesters like the Red-eyed Virco, 
others consistently low like the Catbird, and many others intermedi- 
ate in choice. A Mockingbird nest may be as low as 5 feet or as high 
as 30 feet from the ground. The Mourning Dove usually puts its 
nest 10 to 30 feet above the ground, but some have been found 
lower, even on the ground. The Canada Goose normally nests on the 
ground, but it may use old nests high in trees. Geese have been found 
nesting on cliffs not far from others nesting on river bars along the 
Snake River in Washington. 

The amount of concealment of the nest varies also, even in the 
same species and among closely related ones. No satisfactory rules 
between concealment of nest and conspicuousness of the parents seem 
in order, though possibly some relationship does exist. It has been 
said that where both sexes are equally conspicuously colored, nests 
tend to be little concealed. An evolutionary adjustment between the 
chief enemies, competitors, and elements on the one hand and the 
breeding birds on the other may contribute a large measure of influ- 
ence upon nest location. 

Origin and Evolution of Nesting. Little has been written of the 
possible evolutionary origin of nesting with the exception of song 
and possible exception of bird migration, the most conspicuously 
distinctive breeding characteristic of all animal groups. Because some 
form of nesting occurs among many other and less Righly developed 
vertebrates (reptiles, amphibians, fishes) as well as in invertebrates 
(wasps, ants, spiders), it may well be that nesting antedates the origin 


of the bird itself. One proposal suggests that the use of more elaborate 
structures by birds originated in the advantages over their cold- 
blooded neighbors that insulating or protecting material dragged to 
a nest gave to birds. From this simple beginning developed more 
elaborate structures. But the origin of nests in great antiquity leaves 
us with little to go on save what we can interpret from our knowledge 
of living birds. 


Laying Hours. Birds as a general rule lay their eggs before mid- 
morning (Skutch, 1952). Nocturnal birds appear to reverse the lay- 
ing cycle. Some birds perhaps follow a somewhat modified schedule, 
but the secretive nature or egg laying makes precise information 
rather difficult to obtain. The deposition of the egg may require 
from a few seconds to several minutes at the nest. The most rapid 
laying seems to be that by the Old World Cuckoos and American 
Cowbird, parasitic species and hence ones for which it might be 
unprofitable to linger long in the nest of another (page 108). 

The most common birds lay their eggs about a day apart. Some 
species, however, seem to be consistently different in the laying inter- 
val. The Mourning Dove is said to lay its two eggs about 36 hours 
apart,* an African Swift (Micropus caffer) at 2-day intervals, and the 
Red-beaked Hornbill at intervals of 5 to 7 days (Moreau and Moreau, 

Number of Eggs. The number of eggs (often called "clutch- 
size") varies widely among species and seems related to the kind of 
life. Birds with precocial young lay more eggs than comparable birds 
with altricial young. Evidently the nest and associated parental care 
provide greater security for raising young than the ground or water. 
Within general limits, it is clear, the number of eggs reflects the life 
hazards. Thus migratory species tend to lay more eggs than non- 
migratory ones. But as pointed out in the discussion of breeding po- 
tential (page 247), the number of attempts in the reproductive lifetime 
of the bird probably indicates best the effort needed by the species 
which is to match the mortality rate in the population. 

Among altricial birds, certain definite tendencies may be detected. 
Birds with longer nestling periods lay larger sets than those with 
shorter ones. The number of eggs laid by Passerine birds varies with 
the species, being greatest in the hole-nesters and progressively 
fewer in those with roofed nests, nests in niches, and open nests, 
which trend also parallels the shortened nestling periods (Lack, 1947, 

* Wilson Bulletin, 55:198. 


Birds of the Tropics lay significantly fewer eggs than do their 
relatives in higher latitudes, a general rule believed to apply within 
genera and families as well as within the species. Fringillids of mid- 
dle Europe, for example, average 5.0 eggs to 2.6 for tropical ones, 
Shrikes 5.5 vs. 2.5, and Rails 8.6 vs. 3.8. The increase with latitude 
parallels a tendency for increase from west to east and occurs among 
Passerines, Herons, Hawks, Gallinules, Terns, gallinaceous birds, and 
a few others. This tendency appears absent in Loons, Petrels, Gannets, 
and Doves (Lack, 1947, 1948). The average clutch-size seems to bear 
some relationship to the ability of the parents to provide food in the 
region and in the season. The fact that species having long nestling 
periods lay many eggs (hence have many young to feed) has sug- 
gested the idea that with the same amount of food or food-gathering 
ability available, a large family can be raised slowly or a small one 

The number of eggs laid in a clutch declines in the course of the 
season, though it may rise temporarily in the early part of the nesting 
period of species with long laying seasons. Successive clutches of the 
Wild Turkey have been noted to decline from eighteen to twelve 
and finally to ten eggs. Sets of the Field and Chipping Sparrows (total 
104) averaged 3.39 and 3.66 eggs respectively but declined seasonally, 
as shown in Table 18-1. The number in the Great Tit averaged 10.8 

Table 18-1 
Average Number of Eggs Reported 






Field Sparrow 

, 3.77 




Chipping Sparrow 





Source: Lawrence H. Walkinshaw, "Nesting of the Field Sparrow and Survival 
of the Young," Bird-Banding, 10 (193 9): 107- 114, 149-157. 

before the third week of April and 9.0 after, while that of the Blue 
Tit averaged 11.1 before and 8.4 after (Gibb, 1950). The American 
Goldfinch averaged 5.0 eggs per set during the first half of July, 4.8 
the first half of August, and 3.4 the first half of September. 

Renesting and Multiple Nesting. Among most birds, renesting 
will take place if the nest is destroyed. But such renesting is subject 
to the time of break-up of the nest and the stage of the breeding sea- 
son. If late, renesting usually does not occur. The estimate has been 
ventured that not more than 10 per cent of the actual attempts at 
nesting produce young, and less than 20 per cent of the completed 
ones may endure until the young leave of their own accord (Allen, 


1930). Some birds lay eggs within 5 days of nest destruction, though 
the interval varies in different species and at different stages in their 

Multiple nesting differs from revesting in that it involves nesting 
again in the same season after the successful production of one or 
more broods. Renesting, on the other hand, is the successive attempt 
of the bird to bring off a first brood. The Bob-white very rarely if 
ever raises two broods in a season. Yet a pair whose nests are broken 
up may continue all summer to try to bring off its biological quota of 
one brood. Because a female Bob-white nesting for the first time may 
do so later in the season than older ones, nests of young females may 
be confused with renesting attempts of older birds. 

Incubation Periods. The length of incubation has been discussed 
in Chapter 6, largely from the embryological standpoint (but see 
Table 6 1) . Many inherent difficulties face the field student studying 
incubation periods. For one thing, attentivencss to intubation is not 
consistent among species and individuals alike. Some seem to take up 
incubation duties rather lackadaisically before settling down to regu- 
lar habits. Others faithfully cover the eggs from the start. Among 
species in which only one sex incubates, some time off the nest for 
feeding is necessary. But among those in which the male and female 
share, the eggs may be covered nearly all the time. The incubation 
period for practical purposes should be measured as the interval be- 
tween laying of the last egg in completed sets and hatching of the 
last nestling (Skutch, 1945). But in the Great Tit, "apparent incuba- 
tion" begins any time from 3 days before until 3 days after comple- 
tion of the set (Gibb, 1950). 

In some other birds, such as the Road-runner and Hornbills, incu- 
bation begins with the laying of the first egg, and the young hatch 
several days apart. The significance of this trait has not been deter- 
mined with any satisfaction. In one or two species, only one of a 
pair of differentially hatched young survives. But in most, death of 
the smaller bird probably results from its position of disadvantage in 
a nest of larger ones. But most species of middle and higher latitudes 
begin incubation with the laying of the last egg, or within a day or so 
before or after. 

Careful study at the nest indicates that much coming and going 
may take place among Passerines, not at all like the unmoving vigil 
of a setting hen in the barnyard. The diurnal bird alternates periods 
of attentiveness and inattentiveness throughout the day. Data reported 
for several species show averages in minutes as follows: Allen Hum- 
mingbird on 18.2, off 4.3; Yellow-headed Blackbird on 9.1, off 5.4; 
House Wren on 14.3, off 6.0; Black-headed Grosbeak on 29.2, off 


10.3 (seconds). In all but the Black-headed Grosbeak, incubation is 
by the female only; in the Grosbeak, the male incubated 36.7 per 
cent of the time, the female 62.8 per cent (Weston, 1947). Other 
reported percentages of time during which the eggs were incubated 
are: Song Thrush 58.7, Yellow-headed Blackbird 63.9, Hedge Spar- 
row 66.4, European Nuthatch 73.0, Song Sparrow 76.5, Chiff chaff 
77.5, Ovenbird 82.5, and Marsh Tit 84.0 (Fautin, 1941). But among 
large and robust birds (perhaps also those less highly tensed), pro- 
tracted periods of incubation may be expected. A Buller Molly mauk, 
for example, incubated continuously for 24 days, though a constancy 
of 6 to 1 1 days at a sitting seems to be average for the female (Rich- 
dale, 1951). 

The length of the incubation period obviously varies among spe- 
cies. Among the various factors related to it may be listed the taxo- 
nomic position of the species, the size of the egg, and ecological fac- 
tors (such as dangers to which the nest is exposed) (Skutch, 1945). 

The incubation urge lasts long enough to include the time neces- 
sary for the normal incubation period and a margin of safety. Ordi- 
narily, the urge seems to run out a few days after the expected time 
of hatching. But some seemingly extraordinary constancy to sterile 
eggs has been reported. Evidently a bird could be expected to carry 
on beyond the expected hatching date for as long as a third to a half 
of the incubation period. 

In any discussion of incubation by birds, whether by male, male 
and female, or female alone, one cannot escape mention of unusual 
cases of two groups of birds that depend upon nature to do it for 
them (excluding social parasites). The Brush Turkeys of the Austra- 
lian region bury their eggs in a pile of vegetable matter scratched to- 
gether; the heat produced by decomposition of the plant material 
supplies warmth. They have been found to use sun-warmed sand and 
even volcanic-warmed earth, where they dig a hole 1 to 3 feet across 
and to a similar depth in which a dozen or so eggs are buried. The 
birds may tend the "incubator" up to hatching time. The Black- 
backed Courser of North Africa is said to have taken on the habit of 
burying its eggs in warm sand (Allen, 1925). 

By a special anatomical development, many incubating birds de- 
velop a "brood patch." The feathers on the breast are shed several 
days before laying the first egg and the skin somewhat thickens 
to bring about more intimate contact between the eggs and the warm 
body. The feathers come in again at the fall molt (Barley, 1952). 

Nest and Egg Mortality. It has already been said that a surpris- 
ingly large number of the attempts at nesting fail to produce flying 
young. But this does not mean that a pair may not raise young be- 




cause the first attempt failed, for most common birds try and try 
again. There seems to be no such thing as a nest secure from all disas- 
ters. Even that of the great Bald Eagle may be lost by breaking of the 
tree holding it. A nest in a seemingly impenetrable thorn bush can 
be approached by a more competent animal than man. Yet nests 
vary widely in their degree of safeness, some being more so than 
others. The size of the nest in itself is a protection, as is also the si/c 
of the parents. The firmer the construction, the safer is the nest. 
Placement, concealment, and many additional things influence the 
safety of the nest. Even the size of the colony bears upon the matter 
in colonial species (see Chapter 14). 

In general, nest destruction becomes less frequent toward the 
>oles, toward the dry lands, and toward the higher elevations. The 
last is illustrated by the destruction of 85.7 per cent of the nests in 
the lowlands of Panama compared to 44.8 per cent in the Guatemalan 
highlands (Skutch, 1945). But the destruction of nests and eggs docs 
not necessarily imply a net loss to the bird. An American Robin may 
start to build several nests within a matter of weeks before finally 
raising a brood of young. The percentage of nest destruction thus 
might appear appalling, but one nest built by the pair having suc- 
ceeded, the "pair success" would be 100 per cent. In general, nest suc- 
cess of altricial birds averages about 43 per cent, that of hole-nesters 
about 66 per cent * (see also Chapter 6). 

Many eggs fail to hatch for various reasons, chief among them 
being "infertility." Embryonic deaths occur principally at three 
critical periods, a happening said to be specific for birds. For the 
Domestic Chicken, these fall on the (a) third to fifth, (b) twelfth 
to fourteenth, and (c) eighteenth to twentieth days (Romanoff, 
1949). A study of the Arctic Tern showed a variety of causes for 
nest and egg loss, none of which was a major factor in itself. Of the 
144 eggs laid in 100 nests, 91 hatched and 15.9 per cent fledged young 
(Pettingill, 1939). 

* Bird-Banding, 23:55. 


* ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 


* ALLEN, GLOVER N., Birds and Their Attributes. Francestown, N. H.: Marshal! 

Jones Co., 1925. 

* ARMSTRONG, EDWARD A., Bird Display and Behavior. London: Lindsay Drummond, 

Ltd., 1947. 

* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
BEACH, FRANK, Hormones and Behavior. New York: Paul B. Hoeber, Inc., 1948. 
BULLOUGH, W. S., Vertebra.?? Sexual Cycles. London: Methuen & Co., 1951, 


* DARLING, F. F., Bird Flocks and the Breeding Cycle. Cambridge, England: Cam- 

bridge University Press, 1938. 
HERRICK, FRANCIS HOBART, "Nests and Nest Building in Birds," Journal of Animal 

Behavior, 1(1911): 159-192, 244-277, 335-373. 
*HERRICK, FRANCIS HOBART, Wild Birds at Home. New York: Appleton-Century- 

Crofts, Inc., 1935. 
KENDEIGU, S. CHARLES, Parental Care and Its Evolution in Birds. Illinois Biological 

Monographs, 22 (1952): 1-356. 
LACK, DAVID, The Life of the Robin. London, England: H. F. & G. Withcrby, Ltd., 

LACK, DAVID, "The Significance of Clutch Size," Ibis, 89 (1947): 302-3 52, 90(1948): 

*NiCE, MARGARET MORSK, "Problems of Incubation Periods in North American Birds," 

Condor, 56(1954): 173-197. 

RICHDALE, L. F.., Sexual Behavior in Penguins. Lawrence, Kan.: University of Kan- 
sas Press, 1951. 

* SMITH, STUART, and ERIC HOSKING, Birds Fighting. London: Faber & Faber, Ltd., 


TINBERGEN, N., The Study of Instinct. Oxford, England: Clarendon Press, 1951. 
TINBERGEN, N., Social Behavior in Animals. London: Methucn & Co., Ltd., 1953. 


Young Birds and 
Their Care 

The fact that the male in some species lets the female build the nest 
without his help or incubate the eggs alone afterward may not deter 
him from helping with family duties once the eggs have hatched. 
Sight of the young stimulates the male to start feeding in such cases 
(Skutch, 1953). Only the female Prothonotary Warbler and Field 
Sparrow incubate the eggs, for example, but the males of both species 
care for the young (Walkinshaw, 1938, 1939). In some species, as 
in the American Goldfinch, the male feeds the female while she is 
incubating (though she may get food for herself also), and he may 
help during early brooding of the young, but he participates in their 
feeding later. Some males feed the female who feeds the young or 
who gives food brought by the male to the young. But among 
monogamous birds, the males usually help, though their mates may do 
the greater share of the work. 


Hatching and the Newly Hatched Young. The imprisoned 
young pips the egg by violent exertions of the neck muscles, which 
bring the u egg tooth" atop the end of the bill against the shell until 
a hole opens. The young often alternate periods of activity and 
rest, though some young have been reported to chip the egg all the 
way around in a few minutes of effort. Continued movements of 
the head and body extend the cut until the end of the egg has been 
nearly severed. A few kicks sometimes and the bird has hatched. 
The interval between the pipping of the egg and hatching, however, 



seems to vary from about an hour to several days, depending upon 
the species. Some reported intervals indicate the possible range of 
variability: Sooty Shearwater, 4 days; Woodcock, 36-48 hours; 
American Goldfinch, 15-30 hours; and Ovenbird, 15-20 hours. 

Hatching generally takes place in the forenoon, though some may 
be expected to hatch at any hour of the day (Skutch, 1952). Just 
what regulates such a rhythm for the bird within the protection of 
the nest has not been established. Perhaps the combination of dark- 
ness and light, cooling night temperatures, and the nocturnal lower- - 
ing of body temperature in the incubating bird provides the control. 

The newly hatched bird weighs less than the newly laid egg from 
which it developed, usually by a quarter to a third. The loss in 
weight is attributed to the egg shell and membranes, to dehydration, 
and to gaseous exchange. Because the young bird may get fed rather 
promptly in altricial species, it is seldom known for certain that a 
recently hatched nestling has not been fed. Eight young Great Tits 
definitely known not to have received food averaged 1.30 grams, 
while the egg of the species averages 1.75 grams in weight. The daily 
loss in weight during incubation was reckoned at 0.02 grams (Gibb, 
1950). The newly hatched Blue Tit averaged 0.8 and the eggs 1.1 
grams. Sterile eggs change weight but little, as docs the unincubated 
egg (0.001 grams daily in the Great Tit). The young of the Tits 
weighed 74 per cent of the egg weight; the newly hatched Domestic 
Chicken weighs about 58 per cent of the egg weight. 

The discarded shell may be thrust out of the nest by some young 
birds or left behind by precocial ones. But in altricial birds the job 
of clearing out the shells usually falls on the parents. Many adult 
Passerine birds eat the remnants of shells, but others carry them to a 
little distance from the nest before dropping them. (This act some- 
times leads to the impression that nests have been destroyed or that 
the birds have been robbing nests.) 

The newly hatched young of some species Woodpeckers, Cuck- 
oos, Kingfishers wear no feathers, being completely naked or with 
but a few hairlike feather structures. Young of other altricial birds 
have a few tufts of down (the usual thing for Passerine birds) or have 
the body practically entirely covered with down (as in Hawks, Goat- 
suckers, and Herons). Precocial young carry a coat of down, damp 
at hatching but soon dry and fluffy. The young of some sea birds, 
such as the Gulls and Terns, are rather semialtricial. The chicks stay 
in the nest for some weeks but are covered with down and have about 
as much walking ability as true precocial young. They may roam for 
a few feet about the nest, but this wandering sometimes proves fatal 
to the young when they enter a territory other than that of the par- 


ents. The young of the Brush Turkey show more than the usual 
precocity. It hatches with a good coat of feathers, even of flight 
feathers, which unsheath before the bird reaches the surface of its ^in- 
cubator (page 372). The newly emerged young bird is reported as 
being able to fly. No other newly hatched bird flies at so early an age. 
Altricial young usually have the eyes closed for a number of days, 
a condition varying with the species. The slightest jar on the nest 
(and sometimes sounds) will shortly cause the newly hatched young 
to uncurl the neck, thrust the head upward, and open the mouth for 
food. After the eyes open, the young tend to crouch down into the 
nest under the same circumstances and to respond mostly to the 
parental visit, often as the result of the chirps of the adult. Later they 
may flee the nest when disturbed. The greater the hunger, the 
greater the agitation of nestlings, so that the parent tends to put 
food into the most active and most extended mouth. (For this reason, 
the young Cowbird nestling may gain in the competition for food at 
the expense of the smaller occupants of the nest, which are unlikely to 
present so extended a mouth.) A fed nestling may still open its mouth 
and receive food, but a temporarily satiated bird may not swallow it. 
The parent may remove food not promptly swallowed and give it 
to another open mouth. 

Growth Rate. The growth rate of birds varies widely, depending 
in part upon length of nestling period and size of the bird. The young 
increases its rate of growth a few days after hatching but slows up 
later to give a sigmoid growth curve (Fig. 19' 1). No doubt a great 
many variations occur in nature and from place to place. Among 
some young predators and probably other birds too, a drop in weight 
takes place shortly before departure from the nest. This may be at- 
tributed to loss of fat (perhaps primarily), loss of the feather sheaths, 
and desiccation of the feather pulp. It may be that hardening of 
bone and tissue play a part in weight loss. It may also be that less feed- 
ing by adults occurs at this time, though it seems more probable that 
the loss in weight results from body changes not quickly equalized by 
food and moisture intake. Even in ordinary daily life, the weight of 
the nestling declines slightly during the night but increases again with 
daytime feeding. 

The weight of the young bird may double in the first 2 days, some- 
times for several such intervals. Data for the Field Sparrow illustrate 
this tendency, as well as the tendency for weight decline during the 
night (Table 19-1). The percentage of daily increase also illustrates 
growth rate. The successive percentage increase for the Great Tit 
from the first to the fourteenth day declined as follows (Gibb, 1950) : 
53, 50, 42, 33, 27, 21, 17, 13, 11, 8, 6, 4, 2, 1. 



Pink Smoky Block Growth of "pins' Growth of feothers 

Closed Slit ^pening Dull Clear gray violet Clear gray blue 

G P m 9 Crouching 



Voice changes to lower pitch 

Erupting I 11 2" 3" 



Fig. 19 I. The development of nestling Crows. Figures above the 
data indicate number of birds measured. (After J. T. Emlen, Jr., "Notes 
on a Nesting Colony of Western Crows" Bird-Banding, 13(1 942) :143- 

Table 19-1 
Weight of Young Field Sparrows During First Seven Days 

Age in Days 

in A.M. 


in P.M. 

At hatching 



Source: Lawrence H. Walkinshaw, "Nesting of the Field Sparrow and Survival 
of the Young," Bird-Banding, 10 (1939): 107-1 14, 149-157. 


The various parts of the young bird's body grow at their own rates 
so that changes of the body itself from hatching to adulthood may be 
far from proportional. Comparison of hand-reared and nest-reared 
young show that the old birds are better parents than people. Hence, 
measurements taken from hand-reared young may not be indicative 
of true growth. The primary feathers of precocial birds develop at a 
comparatively faster rate than those of altricial young. Among birds 
in general, the wings develop faster than does the tail, which accounts 
for the bob-tailed look of fledglings during their early flying period. 
The weight may increase from hatching to fledging by ten to twenty 
times. Measurements of weight and other growth in the Scissor- 
tailed Flycatcher showed the following increases from hatching to 
14 days of age (Fitch, 1950): weight 1000 per cent, wing extent 
400 per cent, head length 343 per cent, bill length 300 per cent, 
middle toe length 288 per cent, tarsus length 278 per cent, and head 
width 100 per cent. 

Action of Precocial Young. The precocial chick remains in the 
nest until the down has dried, by which time the bird has become 
strong enough to begin following the adult. The yolk from the egg 
supplies nourishment for a day or two longer than for altricial young. 
Within a day or two of its hatching, the young follows the adult from 
the nest, usually following the female (Grouse, Turkey, Duck), 
sometimes the male and female (Bob-white, Canada Goose, Whistling 
Swan), and rarely the male (Phalarope), according to species. Few 
broods return to the nest after having left it. Until they can fly into 
a tree for roosting like adults (page 208), the young of terrestrial 
birds may roost upon the ground. (Obviously, young of nonfliers 
like the Ostrich would not take to the trees.) Young water birds 
spend the night in the aquatic vegetation, on shore, on platforms of 
some kind, or in other selected places. Adults brood their precocial 
young for some time after hatching, often for several weeks. 

The Impressionable Age. The young bird has a set of instincts 
for action (innate behavior) that unfold as the bird grows older. 
But a number of things can be acquired (imprinting), occasionally 
rather counter to or superimposed upon normal behavior (Thorpe, 
1951). A regular sequence of instinctive actions develop as the young 
bird grows, beginning with first lifting its head and gaping for food 
(see Fig. 19-1). Later the young adds new action begging, crouch- 
ing, preening, signs of fear, bathing, calling. For each species, the 
sequence is about the same in all individuals. 

The young may transfer a normal behavior procedure to some 
other object or action in captivity. Its importance in the life of a 


normal bird seems open to question. The parent association may be 
transferred to human beings or to an inanimate object. A bird fed 
from a pair of forceps may react to the forceps as to the normal par- 
ent. The appropriate reaction of the developing young may be stimu- 
lated by visual (e.g., the forceps), auditory (e.g., call of the parent), 
or tactile (e.g., touch of the parent) releasers. Such reactions are 
never to the environmental situation as a whole, only to parts of it 
(Tinbergen, 1948). But theories of instinctive behavior are open to 
many questions of both interpretation and neurology (Lehrman, 

Fig. 1 9 2. Cardboard dummy that releases positive escape reactions in 
Turkey chicks when sailed to the right (^''Haivk") but not 'when sailed 
to the left (="Goose"). (After N. Tinbergen, "Social Releasers and the 
Experimental Method Required for Their Study" Wilson Bulletin, 60 

The reactions of young to prey have been tested experimentally by 
means of cardboard cut-outs forming a silhouette. When a dummy 
(Fig. 19-2) was sailed over young Turkeys, it caused different re- 
actions, depending upon the direction of movement of the dummy. 
The short-necked appearance of the dummy when moving in one 
direction seemed to signal predator, the long-necked appearance 
the other way seemed to signal Goose or other nonpredator. A 
series of cutout models released escape reactions only when of a short- 
necked shape (Fig. 19*3). Adult and young have rather fixed and 
definite reactions to releasers, though those of the young may differ 
from those of the adult. A young altricial bird will crouch in fear, 
but a precocial bird will "freeze"; in many ways, however, the two 
actions are similar. Later the precocial young may react by escape. 



Fig. 19-4 shows characteristic attitudes of an adult Song Sparrow 
under different circumstances. The nestling may not respond to 
some of these attitudes and to others it may respond only by crouch- 
ing. Other species have various attitudes, some similar to those of the 
Song Sparrow, others markedly different. 

Fig. 19*3. Cardboard models used to test reactions of birds. Only the 
short-necked models (marked -+-) stimulated escape reactions. (After N. 
Tinbergen, "Social Releasers and the Experimental Method Required -for 
Their Study," Wilson Bulletin, 60(1 948) :6-51.) 

Fig. 19-4. Attitudes of a Song Sparrow in alarm, fear, and fright: (a) 
Unalarmed, (b) alert, (c) turning head to look, (d, e) alarm 'with flipping 
of wings and tail, (f) strong alarm, (g) fear, (h) fright (with panting). 
(After Margaret Morse Nice and Joost Ter Pelkwyk, "Enemy Recogni- 
tion by the Song Sparrow, Auk, 58(1941):195-21+.) 


Care of Precocial Young. The young feed some for themselves 
as the brood wanders about. The parent or parents, however, find 
most of the food for the first few days and much of it for an 
indefinite time thereafter. The young recognize many kinds of 
foods instinctively and avoid others in the same way. While many 
acts of the young indicate instinctive controls, others are not so readily 
classified because the adult is along as a model. In general, birds 


hatched in an incubator and later raised in a brooder will grow up 
to act like any other birds of the species, though some readjustment 
of life may be in order as they mature. Yet the appearance of many 
behavior characteristics may be delayed. Young are likely to dust 
earlier in the wild with the brood than when raised artificially. Some 
species have difficulty in recognizing surface water for the first time, 
though most precocial young recognize water drops that simulate 
dew drops, evidently an instinctive response. 

Upon approach of any enemy, the precocial young freeze during 
the first few days but scatter later when they can fly. The warning 
call of the adult becomes recognized quickly, though whether by 
instinct, by learning, or by a combination of instinct and learning 
has not been established. The adult with a brood of young may feign 
injury, though sometimes it will attack an intruder, often a large one. 
Upon the passing of danger, the old bird brings the brood together 
by a special rallying call. During the process of assembling, much 
calling by the adult and peeping by the young may take place. 

The precocial young of aquatic birds leave the nest and swim about 
in the water. Some can dive easily at the approach of danger, but 
many young may freeze when small and escape by flight or swimming 
when older and larger. Many species, perhaps most, use concealment 
more than any other way of escaping danger. 

The number in the brood of precocial birds declines progressively 
during the season because of mortality. Some lost or strayed young 
will join other broods and some recombining occurs among several 
species, especially where broods are concentrated. The average brood 
of the Blue Grouse drops from about four or five in June and early 
July to about two in early August; combining of broods late in 
August may raise the average again (Wing, Beer, and Tidyman, 
1944). The average numbers in Turkey broods from May to Octo- 
ber illustrate a steady decline, greatest when the young are smallest: 
May, 10.7; June, 8.2; August, 7.8; September, 6.9; and October, 6.7 
(Bailey et al., 1948). 

Protection in the Nest. Upon hatching and for some days there- 
after, the old birds brood the newly hatched nestlings to protect them 
from cold or sun. A record of an observer's visits to a Field Sparrow 
nest showed brooding (by the female) during all the visits for 2 days 
after hatching, 87.5 per cent the third day, 60 per cent the fourth, 
20 per cent the sixth, and none thereafter; the young were brooded 
at night 100 per cent of the time until 6 days old (Walkinshaw, 1939). 
Similar variation in brooding as the young become older is the rule 
with most other altricial species, just as incubation may have variations 
(page 371). A comparison of the attentive behavior of eight species 


of New World Warblers during the first 4 days after hatching showed 
brooding periods to average from 15 to 25 minutes in length and the 
time off the nest to average from 4 to 15 minutes (Kendeigh, 1945). 
Among a number of species, one parent, usually the female, stays 
on guard duty for the first days while the other is away, so that they 

Fig. 19 5. The culminating point in the "welcome ceremony" of the 
Yellow-eyed Penguin. The male (left) has just returned and the female 
is at the edge of the nest, the fwo 8-day-old chicks behind her. (By per- 
mission from Sexual Behavior in Penguins, by L. E. Richdale. Copyright, 
1951, University of Kansas Press, Lawrence, Kan.) 

leave the young exposed not at all or momentarily at most. Often 
complicated signals mark the change of watch at the nest, though 
sometimes the birds change places with little ceremony. The mated 
Yellow-eyed Penguins, which recognize each other on sight and some- 
times by voice when out at night, put on a rather violent welcoming 
ceremony, bowing (usually with a burst of notes), and with court- 


ship behavior (Fig. 19-5). In a few species that travel far for feeding 
(e.g., Adelie Penguin), some adults remain as guards or "nursemaids" 
for the half -grown young of several different nests. 

When an intruder disturbs or endangers the nest or young, some 
birds slip away quietly and unobtrusively. Some may attack the in- 
truder, a habit famed in the Eastern Kingbird. Often newspapers re- 
port seemingly unprovoked attacks upon passersby from Screech 
Owls, American Robins, and other birds. The probable explanation 
is presence of young or a nest near by. Other birds may fly about in 
the bushes or trees, often at close quarters. Occasionally, a bird will 
engage in a substitute attack (displacement action) on some object. 
The male American Redstart shows more fear of approaching an in- 
truder than the female. (In most species, the female is the more aggres- 
sive in nest defense.) But he may transfer his attack to her as she 
flutters about the intruder. In some cases, attack, usually by the male, 
may be transferred to an inanimate object. 

Feeding the Young. The growing young require a seemingly 
prodigious amount of food, they develop so rapidly. The young 
have a high protein demand, higher than that for adults. Because 
animal foods tend to be richer in protein (and moisture), more con- 
centrated, and probably more easily made ready for assimilation in 
the body, the parents of many birds, even seed-eaters or herbivorous 
ones like the Grouse, feed animal foods to their young. 

Most birds bring food to the nest in the bill, but some carry it in 
a gular pouch, crop, or sometimes stomach. Predatory birds use the 
feet ("talons"). The parents break up large pieces of food but feed 
small things whole. The Pelican opens its beak and lets the young 
rummage for food. The Bittern regurgitates food directly into the 
mouth of the young, but others may feed it themselves after regurgu- 
tation. Obviously, such food may be in various stages of digestion. 
It is said that as the young grow older, the food is fed in a less digested 
state. Rather unusual among Passerines is the mouth sac of the Gray- 
crowned Rosy Finch of the Alpine zone; it may nest among the rocks 
and have to forage far for insects, which it can bring in numbers by 
means of its special sac (Fig. 19-6). The Pine Grosbeak has an 
identical arrangement (French, 1954). 

The rate at which the parents feed the young depends upon the 
habits of the species, the growth of the young, the success of the 
adults in foraging, and whether one parent or both do the feeding. 
Sample hourly rates reported for the active part of the feeding periods 
show a wide divergence: American Goldfinch 2.0, American Crow 
3.0, Ruby-throated Hummingbird 5.0, Ovenbird 5.3, Scissor- tailed 
Flycatcher 6.7, Yellow-headed Blackbird 9.6. 


More active feeding has been reported, and it may be that among 
some species, much greater activity is the case. The House Wren 
established a record of 77.3 feedings per hour for a day of nearly 
16 hours. Feeding visits numbering from 400 to 600 a day have been 
reported for various other Passerine birds. But large predators feed 
only a few times a day, sometimes only once. The Gray-crowned 
Rosy Finch, because of its pouch (Fig. 19-6) brings food at intervals 
of about three-quarters of an hour; the number of visits in conse- 
quence is about as few per day as those made by any Passerine. Petrels, 

Fig. 19 -6. Skull of a female Rosy Finch to show the left mouth-sac 
(S) with opening in the floor of the mouth lateral to the tongue (T) and 
glottis (G). (After Alden H. Miller , "The Ihwcal Food-Carrying Pouches 
of the Rosy Finch;' Condor, 43 (1941):! 2-7 3.) 

Shearwaters, and some other sea birds feed the young about once a 
day. The Manx Shearwater has been reported to range 600 miles at 
sea from its nest burrow while searching for food. The robust nest- 
lings may go unfed for several days during poor food-gathering 
weather but are able to withstand considerable fasting and yet recover 
lengths without food that would be fatal to the young of almost 
any other species. The young of the Sooty Shearwater spend about 
14 weeks ashore and may also go unfed for various intervals of time. 
During these fasts, the body weight drops but subsequent feeding 
brings quick recovery. One observed young in a colony went unfed 
for 10 days and recovered. Evidently the recovery capacity of these 
young has evolved to balance the variable feeding success of the 
parents during stormy weather at sea. 

Nest Sanitation. The nestlings of common songbirds defecate 
several times a day, sometimes two or three times an hour. A mucous 
coating makes a sort of bag, which the adult carries off and drops. 
For the first few days, the adult may swallow the feces, but later 
usually carries the fecal sack a number of rods away before dropping 


it. Young of predators and most other large birds defecate over the 
edge of the nest; the young of a few Passerine birds (such as the 
American Goldfinch and Barn Swallow) do likewise. 

Length of Nestling Period. The nestling period for practical pur- 
poses is the time between hatching and departure from the nest. The 
nestling period of small altricial birds about equals the incubation 
period. But a few birds show regular variations. Cavity-nesters like the 
Woodpeckers and long- winged flyers like the Swifts and Swallows tend 
to have longer nestling than incubation periods. The young of these 
birds are mature enough when leaving the nest so that most may 
launch out and after a few preliminary wobbles take off in flight 
nearly as well as their parents. Bank-bur rowers (e.g., Kingfisher, 
Motmots, Jacamars, and some Swallows) have both long incubation 
periods and long nestling periods. The nestling period of tropical 
young tends to be longer than that of their relatives in higher lati- 
tudes, which length also parallels differences in incubation periods. 
Although migration, nest building, and other habits of the adults ac- 
celerate poleward, it is not known whether this happens in the growth 
of the young. Some Arctic studies have tended to show that it may 
for some species, but the evidence is fragmentary. 

Some birds have very long nestling periods, especially the larger 
predatory birds and their allies. Young of a large Hawk may spend 8 
weeks in the nest. The young Manx Shearwater spends 6 weeks and 
the Sooty Shearwater 14 weeks in the nest. The Royal Albatross 
takes 7 to 8 months for this period. The little Field Sparrow, on 
the other hand, may leave the nest at 1 week of age, though its incu- 
bation period is 11 days. The Sooty Shearwater, however, has an 
8 -week incubation period and a 14- week nestling time. 


Leaving the Nest. A nestling becomes a fledgling as soon as it 
leaves the nest of its own accord. It remains a fledgling until it can 
fly with ease. (Obviously, some indefiniteness occurs in the use of 
terms.) The young often indicates the approach of its first departure 
from the nest by perching on the edge of the nest for a few hours 
or a few days before leaving. Some young may climb out onto the 
surrounding branches for several days but return to the nest at night. 
Young of some ground-nesting birds like the American Marsh Harrier 
or Black Vulture may move about in the weeds or bushes in the 
vicinity of the nest during the late nestling stage. 

Just as laying and hatching occur largely in the morning, so also 
does departure from the nest. Some young depart by creeping out 


onto the limbs and slowly keeping on going. Some marsh birds (like 
the Yellow-headed Blackbird) leave the nest and creep around in the 
vegetation for several days and even longer before they can fly. But 
some young get up, stretch their wings, and leave by flying, often 
coming to earth after a short distance and then hopping into a hiding 
place. Nestlings within a few days of normal departure may leave 
unceremoniously at the advent of danger, even though unable to fly. 
Many of the baby American Robins hopping about on lawns thus 
may have left prematurely. Young of songbirds dumped out of the 
nest more than 2 or 3 days prior to the normal departure stage are 
hardly likely to survive. 

Feigning Injury. Ornithologists differ in interpreting the meaning 
of injury feigning by birds. Many see it as protective behavior asso- 
ciated with the nest or young and occasionally otherwise (Fig. 
7-18). Others give the bird less credit for its action and interpret it 
as a behavior resulting from psychological conflict in the bird, prob- 
ably from anxiety or fear. This view considers injury feigning as 
related to or actually a form of catalepsy. Birds nesting on or near 
the ground are the chief performers, but seldom other than in con- 
nection with nests or broods (Armstrong, 1947). Experienced field 
ornithologists differ with the view of injury feigning as a cataleptic 
condition, at least in many performers. The evident awareness of 
the acting bird to the intruder and the apparent governing of its acts 
accordingly show more conscious decision by the bird than mere 
catalepsy would involve. The bird's actions appear rather well under 
control and the performer is not often captured by enemies. Injury 
feigning may be a form of diversionary display arising from displace- 
ment of components of other behavior patterns, particularly of threat 
and courtship display. These may have become established as a new 
behavior pattern of survival value (Armstrong, 1949). The simplest 
explanation of injury feigning (and therefore the most reasonable 
one) is that it is merely a highly developed form of conspicuous de- 
parture from the nest (Skutch, 1955). 

Postnestling Care of Altricial Young. The parents ordinarily 
continue feeding the young for some time after they leave the nest; 
seldom, however, are they brooded. Often the parent "leads" the 
newly departed young to a place of safety. By means of calls, 
the hungry young makes known its position to the adults, which by 
that means locate scattered birds to feed them. In some cases and 
among some species, the male takes care of the brood while the fe- 
male lays another set of eggs. Among others, the male may stay for 
a time but leave most of the care to the female. Obviously, this may 
delay or prohibit renesting. 


The length of time during which the parents feed the young seems 
to vary greatly. Full-sized young have been noted begging for food 
weeks after leaving the nest. But others seem able to shift for them- 
selves rather easily. The young often remain in the home vicinity 
for a while before wandering off. Among many of the larger birds, 
a northward wandering has been revealed by banding (see Chapter 
16); but among others, wandering may be an early drift toward 
winter quarters. 

The young of earlier broods in several species have been observed 
to help with feeding the young of later broods, but how regular or 
significant is unknown. It may actually represent a "premature act" 
of the young, just as the young may exhibit other premature acts 
during the fall and winter (page 331). Sometimes there are helpers 
at the nest (Skutch, 1935). 

Mortality of Young. The steady decline in brood size measures 
the mortality of the young in precocial birds, subject to wandering 
and recombining of brood members. The heaviest mortality of pre- 
cocial and altricial birds may occur when the young leave the nest, 
but the crucial hatching period may witness a heavy loss also. Mor- 
tality varies from local catastrophe, when it could reach 100 per cent, 
to very low losses. Probably a mortality of 40 to 75 per cent of the 
young is the most usual range. But complications in gathering data 
on the success of young after they have left the nest make all such 
estimates difficult. The actual success of nestlings (from hatching 
to departure) is often high, locally reaching 90 per cent. Various esti- 
mates of losses of nestlings have been reported; for example, Yellow- 
headed Blackbird 72 per cent, Ovenbird 56 per cent, Kastern Bluebird 
5 1 per cent, and Song Sparrow 40 per cent. Some estimates have indi- 
cated that fully 75 per cent of the young hatched do not survive 
until the first winter. Losses after that have been variously estimated 
up to about 25 per cent by the beginning of the next breeding season 
(see also Chapter 14). 


* ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., Inc., 


* ARMSTRONG, EDWARD A., Bird Life. New York: Oxford University Press, 1950. 
*BENT, ARTHUR CLEVELAND, Life Histories of North American Birds. U. S. National 

Museum Bulletins, 1919-. 
HERRICK, FRANCIS HOBART, Wild Birds at Howe. New York: Appleton-Century- 

Crofts, Inc., 1935. 

HUXLEY, J. S., Problems of Relative Growth. London: Methuen & Co., 1932. 
KENDEIGH, S. CHARLES, Parental Care and Its Evolution in Birds. Illinois Biological 

Monographs, 22 (1952): 1-3 56. 


*LEHRMAN, DANIEL S., "A Critique of Konrad Lorcnz's Theory of Instinctive Be- 
havior," Quarterly Review of Biology, 28(195?):3*7-36*. 

NICE, MARGARET MORSE, Studies in the Life History of the Song Sparrow. Transac- 
tions of the Linncan Society of New York, 4(19*7): 1-247, 6(19W: 1-328. 
*NicE, MARGARET MORSE, The Watcher at the Nest. New York: The Macmillan Co., 

*TINBERGEN, N., ''Social Releasers and the Experimental Method Required for Their 

'Study," Wilson Bulletin, 60 (1938): 6-51. 
*TINBERGEN, N., The Study of Instinct. Oxford, England: Clarendon Press, 1951. 


Heredity in the Bird 

Other than for the adult characters that mark the parents and the 
juvenile ones that mark the young, most parents and offspring ap- 
pear much alike to our eyes. Differences in characters between the 
sexes may be greater than age characters between the respective 
adults and young. Each young bird inherits its genetic composition 
from the genetic make-up of its parents, just as the parents received 
theirs in turn from their parents. The study of inheritance is called 
genetics, but we know little of heredity in wild birds. It may be as- 
sumed, however, that it does not differ significantly from that known 
for domestic and experimental animals. But survival in the wild would 
leave few recipients of unfavorable or weakened hereditary charac- 
ters to be seen. Proportions in the wild may be somewhat distorted 
as compared to experimental animals. 

Inherited Characters. The units believed to control the inheri- 
tance of any feature are the genes, carried in chromosomes and trans- 
mitted through the germ cells. Each gene or pair of genes in homol- 
ogous chromosomes controls the expression of one or more hereditary 
characters subject many times to interaction with other genes, with 
the body, and with the environment. Though a gene controls an 
inherited character, characters may be complex and influenced by 
several genes (multiple factors). Because an individual receives its 
chromosomal make-up from its parents, it normally gets from each 
parent a gene affecting the characters of that parent. The genetic 
constitution of a bird forms its genotype-, the expression of this is the 

One gene may dominate over its corresponding gene and produce 
an individual somatically identical with one that has received identical 
genes from each parent. The one gene is called dominant and the 
other recessive. This condition of differing genes in a pair makes the 



individual heterozygous, that for like genes makes its bearer homozy- 
gous. A widely used example of this occurs in the Domestic Chicken 
in which the gene for rose comb (denoted by R) dominates over the 
gene for single comb (denoted by 7*)- The heterozygous bird (Rr) 
appears identical with the homozygous dominant form (RR) both 
bear rose combs. But the homozygous recessive form (rr) differs 
from both and has a single comb. All offspring of two single-comb 
parents will have single combs also, but all offspring of two 
homozygous rose-comb parents will have rose combs. The offspring 

j o i re* 

of two heterozygous parents (themselves rose comb) will be both 
rose-comb and single-comb birds in the ratio of three to one. The 
three-fourths of the offspring receiving R will have rose combs; the 
one-fourth receiving recessive genes from both parents (rr) will have 
a single comb. 

Characters influenced by the action of more than one gene will 
be inherited differently. Thus, if two genes supplement the action 
of each other, the offspring may differ from either parent accordingly. 
Some characters, such as those of color and size, may be the result 
of several genes having a cumulative action. If a sex chromosome 
carries the gene for a character, the character is sex-linked. Two genes 
on the same chromosome show linkage. But if a chromosome ex- 
changes homologous parts with its fellow at the time of formation of 
germ cells, crossing-over occurs. 

Inheritance in the Wild. Because we know so little of inheritance 
among wild birds, it is difficult to find examples in nature of even 
simple dominance of a normal character over the recessive. Muta- 
tions, however, are likely to be recessive, but mutations in the wild 
are rather unknown and their inheritance factors are less known 
(page 395). Hybrids between species (or races) give some indica- 
tion of the inheritance of interspecific dominant and recessive charac- 
ters. In crosses between Red-shafted and Yellow-shafted Flickers, 
the yellow of the wing and tail feathers appears to be dominant and 
the red recessive (Deakin, 1936). There is some reason also to believe 
that the red of the Red-shafted may be a multiple-factor character. 
Other Yellow-shafted Flicker characters, except the fawn color of 
the throat, appear likewise to be dominant over those of the Red- 

Crosses between members of the Grouse family (Tetraonidae) 
indicate that if one species possesses larger amounts of feathering 
on the legs, this condition will be dominant over less feathering. 
Color in the plumage of birds may be dominant over white in some 
cases and recessive in others. In hybrids between Fringillids, streak- 
ing seems dominant over plain colors. Among hybrids of the Gray- 


headed Junco, Western (Oregon) Junco, and Pink-sided Junco, red 
appears dominant over yellow when the Western Junco crosses with 
the Gray-headed Junco. The yellow dominates over the red, how- 
ever, in crosses of the Gray-headed Junco with the Pink-sided Junco 
(Miller, 1939). 

Crosses between the Golden-winged Warblers and Blue-winged 
Warblers occur in parts of eastern United States and Canada. Hy- 
brids are known as Brewster Warbler and Lawrence Warbler, de- 
pending upon which form they take. The former looks much like a 
Blue-winged Warbler with largely white underparts in place of 
yellow, while the rarer Lawrence Warbler appears somewhat like 
a Golden-winged Warbler with yellow underparts in place of white. 
The Golden-winged when of the pure type appears to be homozygous 
for the dominant white underparts (dominant over the yellow of 
the Blue- winged Warbler), and recessive for the black throat when it 
hybridizes with the Blue-winged Warbler. The Blue-winged when 
of the pure type, on the other hand, appears homozygous for the 
recessive yellow and for the dominant plain throat (dominant over 
black) when it crosses with the Golden- winged Warbler (Pough, 
1946). Because the Brewster Warbler shows the respective dominant 
characters in the hybridization, it may be heterozygous or homozy- 
gous for them. The mating of a heterozygous Brewster Warbler with 
either a heterozygous Golden- winged type or heterozygous Blue- 
winged type can give a Lawrence Warbler (the recessive). Since a 
Brewster may be heterozygous as well as homozygous for the dom- 
inant characters while a Lawrence Warbler must be homozygous for 
the recessive ones, the greater number of Brewster Warblers in the 
wild is evident. 

The single yellow wing bar of the Golden-winged Warbler is 
reported to be incompletely dominant over the double white wing 
bar of the Blue-winged Warbler and linked with the white underparts. 
In the same way, white underparts appear to be incompletely dom- 
inant also over the yellow. Several variations from the "normal" 
Brewster and Lawrence types have been noted, all of which may 
result from this incomplete dominance and perhaps crossing-over 
also (Parkes, 1951). 

Two unusual facts have been uncovered about the hybridizing of 
the two species: (1) Though they occur frequently where the two 
species overlap, few specimens were known before their published 
descriptions in 1874. That they evidence greater abundance since 
1874 than formerly cannot be accounted for entirely by the greater 
interest and zeal of bird people. This has suggested the postulate 
that the ecological and geographical overlap making hybridization pos- 


sible has been brought about by man's disturbance of the biota in 
their joint range. (2) Few cases (none completely free from ques- 
tion) of hybrid-hybrid matings are known, which seems to indicate 
some preferential factor in operation favoring back-cross meltings 
between the hybrids and a parental phenotype. 

"Left-handedness" has been reported in Parrots and may occur in 
other birds, and testing for it seems desirable. Twenty Parrots of 
sixteen forms averaged 72.2 per cent "left-handed" in 380 test obser- 
vations. Eleven showed 75 per cent or more preference in holding 
food with the left foot, five showed 100 per cent preference (Fried- 
mann and Davis, 1938). While nothing is known of inheritance of 
such a character, variation among species would indicate its inheritable 

Many of the morphological, physiological, and psychological 
characters of a genetic nature may be influenced or controlled in 
part by inheritable variations in the nervous and endocrine systems 
controlling them, as well as by the gene structure. Studies of the in- 
heritance of "wildness" in the Turkey show significant differences 
between the adrenal gland and pituitary gland weights of native and 
domestic types (Leopold, 1944). The adrenal weight of the native 
type averaged about 0.03 per cent of the body weight, more than 
twice that of the domestic type. The pituitary of the native type 
averaged about 50 per cent greater than that of the domestic type. 
Since there are marked differences in some manifestations dependent 
upon endocrine glands, the key to the difference in inheritance may 
be in the glands themselves. The brain development, for example, is 
related to adrenal secretions, and the wild type with the larger adrenals 
has a larger brain than the domestic type. It seems clear that con- 
sciously or unconsciously, the less wild and more tractable Turkeys 
have been selected by man. 

Sex Characters. Secondary sexual characters in the form of head 
adornments of wild and domestic types of Turkey gobblers respond 
to hormones also. First-year wild gobblers have little such develop- 
ment in marked contrast to older birds and to first-year domestic 
gobblers. Crosses between the wild and domestic birds result in an 
intermediate condition (Fig. 20-1). The difference seems associated 
with inheritable differences in activity of the endocrine glands, spe- 
cifically the pituitary that controls, through gonadotropins, elabora- 
tion of sex hormones by the gonads. 

Because some characters are sex-linked, the gene for that character 
is carried by the sex chromosomes. Studies of bird and reptile chromo- 
somes show that the male has two similar sex chromosomes (ZZ), 
the female either two differing ones (ZW) or only one (ZO), so that 



-year males V \^ 

First-year males 




Adult males 


Native Hybrid 

Fig. 20 I . Comparative development of male secondary characters on 
the head of native, hybrid, and domestic Turkeys in winter. (After A. 
Starker Leopold, "The Nature of Heritable Wildness in Turkeys" Con- 
dor, 46 (1944):! 3 3-1 91.) 

the condition in birds and reptiles is the opposite of that in mammals. 
The female bird is the heterogametic sex, the male the howogawetic 
sex. Although all families and orders of birds and reptiles have not 
been studied for sex chromosomes, expectations are that the hetero- 
gametic condition in the female is universal among them. 

In addition to sex-linked characters, there occur also sex-limited 
ones. The male plumage of many species, for example, is not a char- 
acter carried in the sex chromosomes but is a character determined by 
the presence or absence of sex hormones. The removal of ovaries 
in birds or their loss by disease will cause the bird to take on male 
characteristics of plumage, voice, and nervous system. Evidently the 
ovarian hormones inhibit the male plumage. In the same way, loss of 
the male hormones, as by castration, disease, or age, will cause the 
male bird to take on a more neutral plumage and action than previ- 
ously. In the light of our present knowledge, it is difficult to distin- 
guish between sex-linked characters (such as the size of males and 
females in some species) and sex-limited or sex-influenced ones 
(such as the male appearance of the individual). Yet the color and 
other sexually dimorphic characters of the Cowbird and Brewer 
Blackbird are genetically determined and not under the control of 
sex hormones (Stanley, 1941). The fact that the female Phalaropes 
and some females of predaceous birds tend to be larger than the males 
may indicate a sex variation perhaps differently inherited. 

Sex Determination. It is customary to say that in heterogametic 
animals, sex determination occurs at the moment of fertilization. But 


it is sometimes said that among animals like the bird in which the 
male is the homogametic sex and all sperm alike, sex determination takes 
place a few moments prior to the fertilization when the first polar 
body leaves the oocyte ahead of the rupturing of the ovarian follicle 
to release the oocyte. If the released oocyte itself carries the sex 
chromosomes, the egg will give rise to a male individual, but if the 
sex chromosome passes to the polar body, a female will result. 

Just how the sex chromosome gives rise to a male is not clear, but 
presumably it carries potentialities for maleness that modify those of 
the autosowes so that maleness predominates and a male bird results. 
The male shows a higher metabolic rate than the female, which seems 
an associate of maleness. 

The sex ratio in Chickens appears to change with increased pro- 
duction of eggs so that later eggs produce more males as the energy 
resources of the laying bird become depleted (Romanoff and Roma- 
noff, 1949). The sex ratio of 589 first-laid of paired Rock Dove eggs 
was 103 (103:100) and for 545 second-laid of paired eggs, 113 
(113:100) (Cole and Kirkpatrick, 1915). 

The ratio of male and female individuals at fertilization is referred 
to as the primary sex ratio (Chapter 8). On theoretical grounds, this 
should always be even that is, equal numbers of ovums with and 
without the sex chromosomes are fertilized. So far as is known, birds 
produce both types of gametes (Z and IF, sometimes Z and O) in 
equal numbers. There is no evidence that one type of ovum attracts 
sperm more than the other, though such could conceivably occur. 

Lethal Characters. Experimental crossing of Domestic Chickens 
has demonstrated the existence of lethal character's and that individuals 
having them die in the embryonic state, seldom living after hatching. 
The genes for them may be carried in the sex chromosomes, so that 
the lethal characters appear in one sex more than in the other or 
exclusively in one sex. Some of the failures of eggs to hatch in the 
nests of wild birds might very well be caused by lethal characters, and 
examination of such eggs would presumably clarify this point. 

Mutations. Gene changes that result in new inheritable variations 
have been termed mutations. The mutation rate seems to be low but 
variable among species (as should be expected). Mutations are not 
well known for wild animals, in part because mutations tend to be 
recessive to the normal and in part because they tend to be of less 
survival value to the individual. In addition, it may be that mutations 
are but part of an upset in the body complex (Lee and Keeler, 1951). 

A red condition, possibly a mutation, appeared among Bob-whites 
in the Southeast and persisted in the wild for some time but slowly 


disappeared (page 126). Studies and experimental breeding of cap- 
tured birds established that the birds with red plumage were distinctly 
weaker, less thrifty, and less vigorous than the normal. The red con- 
dition was apparently an incomplete dominant (Cole, Stoddard, and 
Komarek, 1949). In general, however, normal red phases in nature 
(page 126) do not mean weaker individuals, at least so far as known 
to biologists (as in Screech Owls, Pygmy Owls, and Ruffed 6 rouse), 
which suggests the possibility that red mutations might eventually 
become normal colors if the vigor of the bird should be unimpaired 
or re-established. 

Albinistic characters seem to be inherited as simple recessives. Yet 
normal white, as distinguished from albinism, may be dominant to 
color. An 18-year series of observations of a flock of House Sparrows 
in Washington, D. C., is interesting in shdwing persistence of albinism. 
It is also interesting in showing how easy it is for alert ornithologists 
to discover new things in the bird world. The flock contained many 
partially albinistic members through the years, and they showed lack 
of vigor as compared to other flocks of Sparrows (Davis, 1947). The 
persistence of white is of greatest importance perhaps as showing the 
flock persistence through several generations during which individuals 
were hatched and died. 

Melanism (and occasionally another color) appears as a normal, 
sometimes dominant, color phase in several species of birds (page 
126). It may also occur as a mutation of part or all of the plumage. 

Dilution of color probably constitutes a form of partial albinism. 
Birds are seen occasionally that have a paler color than the normal, 
such as a paler blue condition in several of the Jays, that suggests 
decrease of the blue (a structural color, page 124) by increase of the 
white component in the color. The intensification of color appears 
also in some birds, which suggests a reduction in the white of the color 
or perhaps increase of melanins. The dilution and intensification in 
the plumage may occur by change in the white, just as a painter 
makes shades by adding or reducing white. 

Darkening of the plumage color appears also, and may indicate 
partial melanism, insufficient to show as deep brown or black. Melan- 
ism and albinism appear more often or perhaps are recognized more 
often in birds respectively of predominantly grayish or blackish plum- 
age. Other changes also probably sometimes mutational in origin are 
the yellowish appearance of normally greenish plumages of some 
Parrots (page 126). Most of the important characters of animals seem 
to result from multiple gene action, so that this yellowish appearance 
may be a mutation in one or more of the genes involved. In the 
South American genus Buarremon, a mutation of the dark pectoral 



band is postulated as giving rise to the species Buarrenwn mornatm 
from Bitarrewon bruwneinucha. The former now occupies a restricted 
range in the subtropical zone of western Ecuador, while its parent 
species ranges widely. 

A mutation to be inherited must be in the genes of the germ cells; 
mutations in the somatic cells would not be inherited. Our best evi- 
dence of mutation in birds appears in Domestic Chickens and Domestic 
Pigeons. Many bizarre varieties and individuals have resulted from 
mutations and recombinations, some of them markedly different from 
anything in the ancestral Jungle Fowl or Rock Dove. 

Geographical and Ecological Variations in Inheritance. While 
we recognize many geographical variations, some over large areas 
(such as the paler plumage of desert birds or darker plumage of humid 
region ones), some over small areas (such as the appearance of white 
feathers in the birds of some localities), we are unable to describe 
clearly their genetics. Yet a genetic base seems certain. The many 
geographical variations recognized as subspecies attest to probable 
inheritance of characters on a regional basis. Ecological variations also 
show probable regional inheritance, as in the reported possession of 
longer wings by Starlings in the windy Faroes as compared to the less 
windy mainland. The increased size of birds in the cold regions and 
decreased size in the warmer ones in accordance with the Bergiiiann 
Rule (Chapter 10) likewise indicates ecological and geographical 
variation of an inherited nature. 

Segregation of plumage characters in Old and New World groups 
of species in the genus Colinnba shows that genes affecting plumage 
color are distributed in significantly different frequencies (Fig. 20-2). 
Antigens in the blood also show geographical segregation. 

Characters shared 
between few Old a 


Old a New World 




Old a New World 


Fig. 20 2. Categories of plumage characters in the genus Columba. 
(A) exclusive to Old World birds, (B) mostly of Old World birds, (C) 
common to both Old and Neiv World birds, (D) mostly of New World 
birds, (E) exclusive to New World birds. (After Russell W. Cumley 
and Leon J. Cole, "Differentiation of Old and Neiv World Species of the 
Genus Columba," American Naturalist, 16(1942):510-58l.) 


Aberrant Forms in Nature. Many strange or grotesque individuals 
have been reported by collectors, bird banders, and others. Crossing 
of the mandibles or greater length than normal has been reported 
many times. How many of these are inherited as mutations or how 
many have resulted from deforming accidents or some other ab- 
normality, we do not know. But the presence of similar conditions 
in the bills of domestic poultry and the hereditary nature of 'some of 
them suggests the possibility that some could be inheritable in the 
wild. Abnormalities of the feet occur, but most of these may be the 
result of disease or accident. Major abnormalities of the wings most 
likely would have an unfavorable survival value and the malformed 
individual thereupon be eliminated. This may account for the rarity 
of wing abnormalities compared to those of the feet or bills. Yet 
many injured birds recover (Chapter 21). 

Natural Hybrids. In addition to the hybrids cited earlier in this 
chapter, many apparent hybrid individuals have been found. The 
Blue and Snow Geese have been reported to cross freely in the north- 
ern Hudson's Bay region; but the explanation may be that the Blue 
Goose is a geographically restricted color phase of the Lesser Snow 

Hybrids occur between various Ducks (and sometimes between 
Geese) in Waterfowl marshes, perhaps because of the pressure for 
space and mates within the restricted area of the marsh habitat. The 
Mallard is especially likely to cross with other Ducks; so also is the 
Pintail. In general, hybridization occurs most frequently between 
closely related or ecologically closely associated species, particularly 
where the two sexes do not form a pair-bond of any duration or care 
for the young jointly. Presumably, the complex courtship and family 
procedures (with resulting lengthy mated life) together militate 
against the crossing of two dissimilar individuals, as in two birds of 
different species. Though nearly all orders have shown at least one 
hybrid, judging by numbers reported, they occur most often in the 
Ducks, gallinaceous birds, and Passerines. They occur less often 
among Pigeons, Hummingbirds, Hawks, and Woodpeckers. 

Use of Birds in Genetic Studies. Geneticists and animal breeders 
have found the Domestic Chicken and Domestic Pigeon particularly 
plastic in their hands. Though the best evidence indicates that these 
two came from the Jungle Fowl and Rock Dove, respectively, the 
probabilities arc -always present that man has added a sprinkling of 
genes from other species during the long history of domestication. 
The Canary has also proved rather amenable to breeding and is fre- 
quently used in the genetics aviary. Other domestic birds have proved 


less pliable in the hands of the breeder, though several breeds of some 
have been developed, either by modern or ancient man. 

Geneticists use the Pigeon and Chicken for breeding experiments, 
for which they have served well. About seventy-five characters, each 
determined by a single gene, have been identified or their inheritance 
determined in the Chicken. A large number of quantitative char- 
acters, such as of size, weight, age of laying, number of eggs, and the 
like, have been studied genetically in the Chicken and Pigeon (Snyder, 
1951). Many multiple characters are likewise known. Thus, the 
normal dark color of the Dove, Streptopetia chhievsis, forms a triple 
allelomorph with the sex-linked "blend" and "white" of the captive 
Ring Dove (Cole, 1930). 


DOBZHANSKY, T., Genetics and the Origin of Species. New York: Columbia Uni- 
versity Press, 1941. 

FISHER, R. A., The Genetical Theory of Natural Selection. Oxford, England: 
Clarendon Press, 1930. 

HUTT, F. B., Genetics of the Fowl. New York: McGraw-Hill Book Co., Inc., 1949. 

MAYR, KRNST, Systematic* and the Origin of Species. New York: Columbia Uni- 
versity Press, 1942. 

John Wiley & Sons, Inc., 1949. 

SINNOTT, E. W., L. C. DUNN, and T. DOB/HANSKY, Principles of Genetics. New 

York: McGraw-Hill Book Co., Inc., 1950. 
*SNYDER, LAURENCE H., Principles of Heredity. Boston: D. C. Heath & Co., 1951. 


Health in Wild Birds 

Because birds like other free-living organisms must be their own 
providers, the price of body malfunctioning is likely to be high. The 
kinds of malfunction are many (e.g., endocrinal and nutritional dis- 
turbances, injuries, and environmental strains). Diseases of birds, 
especially the parasites to which birds are hosts, have been frequently 
reported. Diseases in wild birds, so far as known, seldom limit the 
populations except possibly locally, though widespread disease in birds 
could conceivably cause extensive mortality. Heavy losses from botu- 
lism in Waterfowl areas of western North America have occurred. 
Malfunction of the body through external strains such as of weather 
or diet is a more likely health condition affecting bird numbers in 
cooler or drier regions. In general, diseases become more important 
in the warmer and wetter regions, climate in the cooler and drier 
ones. Malfunctions of the body arise from causes that may be listed 
under five headings: 

Poor living habits (poor habitat, fatigue, disturbance) 

Disease organisms (parasites, bacteria, viruses, rickettsias, fungi) 

Toxins and poisons 

Dietary deficiencies (lack of vitamins, minerals, water) 



Once choice territory or living range has been taken up or oc- 
cupied, birds unfortunate in finding good range are likely to be 
forced into poor quality habitat, submarginal for the species (Chap- 
ters 1 1 and 12). Little study has been made of ill-situated birds, except 
for a few game species. But studies indicate that such birds live a 
rather more strained kind of life than well-situated ones. Birds banded 



in some environments have been reported to outweigh some in other 
environments, which indicates a probable relationship, difficult to 
evaluate though it may be. Bob-whites on poor range are more likely 
to lose weight (a fairly good indicator of body health) and numbers 
in severe winters than are those on good range. The loss of birds 
from unseasonable storm, as has been reported innumerable times both 
in the winter range and during migration, testifies further to drain 
upon the bird and its energy resources. Birds in smokey cities have 
been noted to have soot in the air passages, just as do many human 
beings. Those in areas of dust storms presumably would breathe in 
some dust. Ground birds are exposed to more blowing dust than an 
animal like man breathing air from somewhat higher above the ground. 


Internal Parasites. Regular internal parasites of birds belong to 
five groups: Protozoa, Platyhelminthcs, Nemathelminthes (Nema- 
toda), Acanthocephala, and Arthropoda. The protozoan parasites of 
birds belong to the classes Mastigophora (Flagellates), Sarcodina 
(Amoeba), and Sporozoa (Herman, 1944). They occur mostly as 
blood and intestinal parasites, but some may be found in the muscle 
(e.g., Sarcocystis) and elsewhere in the body. 

In a study of blood smears of eighty-one species of birds, about 
one-third showed blood parasites (Huff, 1939). A similar study on 
Cape Cod of 2,385 birds of sixty-one species showed blood parasites 
in 269 birds of twenty-seven species, the highest rate of infection 
being 60 per cent in the Chipping Sparrows (Herman, 1938). Blood 
smears of birds in general show varying amounts of infection from 
about 3 per cent to 100 per cent, depending upon the size of the 
sample and the time it was taken. Because in some diseases the acute 
stage when parasites can be detected easily in blood smears lasts but 
a short time, a higher incidence in birds than usually shown may be 
possible, perhaps as much as 50 per cent in marsh birds. Most of the 
protozoan blood parasites are malarial or are malaria-like in their 
action. Those commonly reported for birds are Haemoproteus, 
Leucocytozoon, and Plaswodium. Blood parasites spread to birds by 
means of insect intermediate hosts. Birds once infected by a malarial 
organism seem usually to be immune from reinfection by the same 
parasite. There are indications that the infection by a parasite may 
give some degree of immunity from other and especially closely allied 
malarias; in some cases, however, this is definitely not so. 

Most protozoan intestinal parasites of birds belong to the order 
Coccidia. Two genera occur rather commonly in birds, Isospora and 


Eimeria. A tendency has been noted for Isospora to be found in the 
taxonomically higher forms of birds and for Eimeria to infect the 
lower ones. These organisms cause the diseases known as coccidiosis 
by growing in the epithelial cells of the digestive tract, usually in the 
lining of the intestines. A high percentage of some birds (60 to 100 
per cent of specimens in some studies) appears to be infected, but the 
balance between host and parasites seems such that birds show few ill 
effects. Yet one must assume, with considerable good reason, that a 
bird with them might suffer or be weakened by them when conditions 
for bodily vigor arc poor. Birds with acute coccidiosis may show loss 
of appetite, emaciation, droopiness, and diarrhea, often followed by 
death. Birds become infected by picking up spores, usually v with their 

A number of flagellate parasites have been found in blood and 
intestines. Those in the blood are chiefly Trypanosoma-, those of the 
digestive tract, usually in the intestines, are chiefly species of Tri- 
chomoTjas. The latter genus has been reported in small numbers and 
species from about half the orders of birds. Trypanosoma seems to 
be spread by arthropod intermediate hosts, Tricbomonas by ingestion. 

Cestodes (tapeworms) have been reported from many birds; they 
belong to several different genera. Indications point to a greater 
abundance in birds than mammals, possibly because cestodes of birds 
have a more marked host specificity than those of mammals. Several 
studies have indicated a greater incidence or menace of tapeworms to 
young birds than to adults. The incidence of parasitism by tape- 
worms has varied widely as reported in different investigations, from 
almost nothing to as high as 61 per cent. One young Blue Grouse is 
reported to have had ninety-three tapeworms that almost completely 
filled the intestines. Tapeworms need an intermediate host, and those 
of many common birds develop in insects from which the birds get 
them by feeding upon infected insects. Others develop in snails or 
crustaceans. Diphyllobotbrmm oblongatum develops in a fish that 
may be caught by a Gull and eaten by it or fed to its young, which 
would infect the adult or young. 

Trematodes (flukes) are mostly internal parasites, though a few 
may form cysts in the skin. There are a large number of flukes para- 
sitic upon birds, especially water birds. They may be found in the 
intestines, veins and ducts of the liver and pancreas, oviduct, bursa 
of Fabricius, caecums, trachea, kidney, and blood stream. Many of 
the flukes have as an intermediate host snails, crustaceans, aquatic 
insect larvae, fish, or tadpoles (Fig. 21-1). The cercarial stage of bird 
flukes (Shistosoma) may be one of the organisms involved in "swim- 
mer's itch" (Worth, 1949b). 



Fig. 21 I. Life cycle of a trematode. Water birds hi fee ted with flukes 
way contaminate the water and in turn inject the dragonfly nymphs 
from which the chain passes to a tadpole eaten by a water bird, which be- 
comes infected with flukes. (By permission from Bird Life, by Edward 
A. Armstrong, p. 139. Copyright, 1950, Oxford University Press, New 

A very great number of nematodes (roundworms) have been re- 
ported from birds. They have been found in the proventriculus, giz- 
zard, trachea, crop, intestines, caecums, eyes, and body cavity. Micro- 
filaria have been found in the blood stream of a number of birds. Many 
nematodes have direct life cycles, and the birds become infected by 
feeding upon contaminated soil. Others require an intermediate host, 
such as a grasshopper, from which the birds get the parasites by eating 
infected insects. The caecal worm, Heterakis gallinae, carries the pro- 


tozoan Hist onion as welea%ridis that causes the disease known as 

Several spiny-headed worms (Acanthocephala) have been found 
in the intestines of birds. The intermediate hosts are crustaceans and 
insects from which the birds get the parasites by feeding upon in- 
fected animals. 

Parasitism by Reigbardia has been found in Gulls and Terns. The 
parasites belong to Pcntastomida, wormlike animals of uncertain 
systematic position but usually placed in the phylum Arthropoda. 

External Parasites. External parasites of birds belong to the 
phyla Annelida and Arthropoda. The former contains several leeches 
(Hmmdinea) reported from water birds. Almost all other external 
parasites belong to the Arthropoda. 

Parasitic arthropods of birds are of two classes, Arachnoidea 
(Acarina: ticks and mites) and Insecta (Mallophaga: biting lice; 
Dipteria: mosquitoes, black flies, bottle flies, hippoboscid flies; Sipho- 
naptera: fleas). Tt is said to be possible for every kind of bird to have 
at least three kinds of lice, one or two hippoboscids, and several mites. 
Those feeding or alighting upon the ground may carry one or more 
kinds of ticks (Peters, 1936). In 239 species of eastern birds, ecto- 
parasites were reported as follows (which may indicate relative dis- 
tribution): lice 220, mites 61, hippoboscid flics 56, and ticks, 55. 

Ticks and mites are external parasites living in the feathers or bur- 
rowing into the foot scales of birds or attaching temporarily for 
engorging on the blood of the host. During the season when ticks are 
most active, ground birds and those living in thickets may become 
heavily parasitized with ticks. Ruffed Grouse have sometimes been 
reported with head and neck literally covered with ticks. A red-eyed 
Towhee has been found with twenty-nine ticks attached to the head. 
In the winter grounds of the southern states, 10 to 20 per cent of 
ground birds caught in banding traps may carry ticks. 

Fleas have been reported from birds, but little is known as to their 
harmfulness or role in transmission of disease. 

Mallophaga as a rule live at the base of feathers. Some live off the 
feather parts and others from skin particles. A few feed upon the 
blood of the host and even upon the fluids of the eyes. Eggs are 
usually laid among the feathers. Fiagetiella live among the feathers of 
Cormorants and Pelicans and even in the mouth, where they suck 
blood. They have specially adapted tracheas for their life in aquatic 
surroundings. Parasites are highly host specific, and parasite groups 
have large numbers of species. There are about 2,100 known species 
of Mallophaga, for example, which would suggest a total of 10,000 
for all species of birds (Eichler, 1946). 


Mosquitoes parasitize birds and the culicine mosquitoes transmit 
avian malaria (Plasmodium) . Blackflies have been found to transmit 
Leucocytozoon malaria; hippoboscid flies transmit Haeinoprote'iis. 
Mosquitoes (perhaps blackflies also) carry fowl pox. Hippoboscid flies, 
unlike mosquitoes and blackflies, spend their whole life cycle on birds 
or in the debris of nests. They are blood-sucking insects but are 
not known to cause ill health. Ticks may transmit tularemia to birds. 

Nest Parasites. Studies of nests of birds have shown a surpris- 
ingly large fauna of insects and other animals, mostly harmless. But 
some nest parasites may be serious enemies of the nestlings. As many 
as a billion mites have been estimated for a nest box; such mites can 
be destroyed by creosote or kerosene. 

The common nest parasites are maggots of flies (Apaiilina, Calliph- 
ora, ProtocalUphora) belonging to the fly family (Musidae) though 
sometimes listed as a family of their own, Calliphoridae. The fly lays 
eggs in nests of birds, and those of the Tree Swallow and Eastern 
Bluebird have been reported as particularly likely to be parasitized. 
Any nest, however, especially those in cavities and nest boxes, may 
be parasitized. Nests of sticks, like those of the House Wren, seem 
less desirable as an abode for parasites than those of Swallows. The 
larvae attack the nestlings, usually about the legs and feet, mostly at 
night, and retreat into the nest material in the daytime. In a study in 
New England, 94 per cent of the Bluebird and 82 per cent of the Tree 
Swallow nests were parasitized (Mason, 1944). In inclement weather 
when little food may be brought to the young, the attacks of the 
parasites upon weakened nestlings may cause death. More than 400 
of the parasites were found in the nest of a Tree Swallow, but the 
more usual number when present seems to be from five to about sixty. 

Host and Parasite Adjustment. Normal parasites and their hosts 
have evolved together and seem well adapted to each other. Closely 
related parasites evidence close relationship of their respective hosts 
(Fahrenhoh Rule). If two groups of birds have three genera of 
Mallophaga in common, their close relationship is deemed established 
(Hopkins, 1942). Thus the Mallophaga of the Ducks, Geese, and 
Swans suggest that Geese and Swans form a distinct group, the 
Ducks another. Those of the Hummingbirds suggest a closer rela- 
tionship to the Passerine birds than to the Swifts. The Herons and 
Storks may be more distantly related than assumed; parasites suggest 
that the Herons arose in South America and the Storks in Africa. 

Predators often obtain temporary infections of parasites from their 
prey. In some cases, predators have appeared heavily parasitized, as 
though they served to concentrate the parasites of their prey. Flying 


ectoparasites like the hippoboscid flies would seem to have little diffi- 
culty in transferring from one bird to another, though they seem to 
spend most of their time deep in the feather coat. Flightless ecto- 
parasites have several ways of transmission open to them. They may 
pass during direct contact, as between parents and nestlings or 
between birds in roosting flocks. Parasites attached to objects, such 
as branches, loose feathers, or nest material, may pass to another bird 
touching the objects. Some may transfer during free-living stages or 
by direct attachment such as of ticks and mites. Smaller parasites 
attached to a hippoboscid fly or other larger parasite could be trans- 
ferred with the temporary carrier. Ectoparasites of prey species can 
be transferred to a predator or its young during feeding. 


Virus Diseases. Probably the best-known virus disease of birds 
is psittacosis (page 407) because it on occasion may be transmitted 
to man (first known case in Switzerland, 1879). Seldom is it trans- 
mitted from one human to another. It has been reported from Parrots, 
Domestic Chickens, and Domestic Pigeons. A closely related virus 
has been reported from the Atlantic Fulmar. 

Flies have been shown to carry the virus for ulcerative enteritis, 
and birds may become infected by eating infected flies. But it is more 
often spread by fecal contamination. Cases have been reported of a 
few birds (Pheasants, Pigeons, House Sparrows, Storks, and others 
likely to be found about farm yards) becoming infected with the 
virus responsible for sleeping sickness of horses (equine encephalitis). 
It has been found in newly hatched domestic chicks, which indicates 
that it can be egg-borne. 

An avian cancer-like condition suggesting a filterable virus as the 
responsible organism and presumably transmitted by biting flies has 
been suggested as a disease found in a British Columbia Blue Grouse. 

Bacterial Disease. The best-known bacterial disease of birds is 
the botulism sometimes called "western duck sickness." It has oc- 
curred a number of times in western water areas where conditions 
may be favorable at times. The causative organism has been de- 
termined as Clostridiwn botulimis, Type C. It develops in decaying 
organic matter in warm, moderately alkaline waters. For this reason, 
it seldom occurs in forested areas, where the waters are usually acid. 
Birds become sick by drinking water containing toxins produced by 
the bacteria. If taken in dilute amounts, a sick bird usually recovers, 
but continued or concentrated ingestion usually proves fatal. Great 


losses of Waterfowl and other birds have occurred in western waters 
(Kalmbach, 1934). 

Birds feeding upon decayed meat, or upon maggots that have fed 
upon it, have become infected with a form of botulism. The body 
fluids of Vultures, however, seem to have antibiotic factors of un- 
known nature, so that they are resistant to disease organisms, toxins, 
and ptomaines harmful to most other birds or other animals. Bacteria 
(Pasturella), thought to be transmitted by mites, cause bird cholera. 
Ruffed Grouse and some other birds have had tularemia (caused by 
Pasturella tularemis). Avian tuberculosis has been found frequent 
among birds in contact with poultry, such as the House Sparrow, or 
among those eating refuse, such as the Gulls. A related bacterial 
disease has been reported as "pseudotuberculosis." Bird pneumonia 
also occurs. 

Fungus Diseases. How widespread fungus diseases may be in 
wild birds is unknown, and the chances are that they are not common. 
Fungus diseases have been found often among 7.00 birds and sometimes 
among poultry. Possibly some of the avian tuberculosis or pneumonia- 
like diseases have been caused by fungi. Aspergilloms has been found 
pathogenic in a number of birds, such as Gulls, Ducks, Geese, Hawks, 
and Owls. Though usually found in the lungs, it has been reported 
also in the esophagus, viscera, and body cavity. 

Birds, especially the Vultures, have been accused of spreading 
anthrax in domestic livestock areas, but there is no reliable evidence 
to indicate this. (There is also no evidence to indicate that birds may 
spread hog cholera or hoof and mouth disease.) 


Because birds have a high body temperature, few of the diseases of 
man attack them. Just as a high fever may destroy invading organisms 
in man, the fever-high temperature of birds seems to protect them. 
Conversely, few avian diseases attack man. An exception is the disease 
commonly called "Parrot fever" but known to medical science as 
psittacosis. (But because Pigeons in city parks may carry it also, it 
sometimes goes by the name of ormthocosis.) Prohibition upon im- 
portations of Parrots into the United States stems from the dangers 
of psittacosis, a highly fatal disease. 

No wild bird of the United States and Canada is known to serve 
as the reservoir of any human disease. Birds have malarias of their 
own (some fifteen kinds) but none attacks humans. Yet because of 
bird malaria, birds (Canary, Chicken, Duck, Pigeon) have been used 


in testing new malarial control drugs (e.g., atabrine, plasmochin, palu- 
drine, pentaquin, chloroquin) before trying them on human beings. 
Birds have also been used to study the life cycles of malarial parasites, 
particularly their "exoerythrocytic" stages. 

A number of cases of birds becoming infected with diseases of 
domestic animals, particularly poultry diseases, have been recorded. 
But the chances of a human being getting a disease from any* wild bird 
are practically nil. The chances of domestic stock becoming in- 
fected from wild birds are just about as low. 


Poisons. Birds have been found sick from a number of poisons 
besides insecticides (page 445). From time to time, American Robins 
and Cedar Waxwings feeding upon fermented fruit have been influ- 
enced by the alcohol of fermentation, showing all the signs of 
intoxication. "Intoxicated Robins" sometimes become the basis for 
newspaper stories and pictures. 

Various alkalis in the water of dry regions if in sufficient concen- 
tration can cause severe irritation of the intestinal tract. Alkali poi- 
soning, however, has been confused with both lead poisoning and 
botulism, so that it is not clearly understood. 

Among some Waterfowl, the most serious single source of injury 
to birds outside of hunting itself is the danger of lead poisoning. Lead 
shot sinks to the bottom of lakes, streams, and marshes, where it is 
picked up by the feeding birds. The shots remain in the gizzard 
(because of gravity), where they may be ground up to be absorbed. 
Because lead is a cumulative poison, the number of shots in the gizzard 
may be few and yet the bird may be affected. From one to six No. 6 
lead shot have proved fatal. There is no known treatment (aside from 
feeding and watering in pens and from "stomach cleaning"). Nearly 
all Waterfowl hunting areas are to Waterfowl a source of danger 
from lead poisoning. Many tons of shot have been fired over small 
areas. Such lead shot in water tributary to domestic water supplies is 
a source of contamination and of concern to health authorities as a 
menace to public health. 

Birds have been poisoned by feeding upon insects killed by insecti- 
cides. They have been poisoned on many occasions in the course of 
widespread poison campaigns for rodent and predator destruction. 
Some studies have shown that insecticide sprays in low concentrations 
may control insect outbreaks without killing off the bird life. But of 
the effects from newer insecticides, little is known (page 446). 


Injuries. Injuries of various kinds occur among birds. The one- 
legged Sandpipers occasionally seen along shores, for example, per- 
haps lost the other foot in an accidental encounter with a clam or 
other aquatic organism. Many accidents can happen to cause the loss 
or crippling of the feet in birds. One-legged Robins and other birds 
of the garden have been reported many times. 

Birds have a remarkable ability to dodge twigs and other projec- 
tions, but sometimes they fail to do so in flight and injury may result. 
A number of Ruffed Grouse have been reported with pieces of broken 
sticks completely imbedded in the body, yet the bird recovered. 
Head injuries usually result in death, probably always if the skull is 
fractured. Ptarmigan of the Arctic tundra are reported to be killed 
frequently by flying into telegraph wires, sometimes being completely 
decapitated. But the largest number of injuries probably occurs along 
highways as the result of collision with automobiles, some highways 
being littered with dead birds. This is especially true during the 
nesting season. Injuries at lighthouses have long been reported. Frac- 
tured bones may heal within a few days if the bird or bone remains 
motionless. Even wing bones have healed sufficiently after fractures 
to become usable again, though most broken- winged birds arc doomed 
(Wood, 1941). Most birds recovering from wing fractures have had 
only the radius or ulna broken, so that the second member of the pair 
prevented displacement by muscle tension. 

Freezing of the feet and other parts of the body ordinarily seldom 
happens, though the soft toes of Mourning Doves seem especially 
likely to become frozen in severe weather. 

The injurious effect of oil pollution has brought disaster to birds 
on a number of occasions, both inland and along oceans. Oil mats the 
feathers and afflicted birds become water-soaked or chilled rather 
quickly. Oils may have a toxic effect also. Keeping oil from getting 
into the water is the only known means of preventing oil pollution 
and consequent injury to water birds from it. 

Bathing. Many ground birds, like the Domestic Chicken or the 
Vesper Sparrow, maintain cleanliness by bathing in the dust. It may 
help control ectoparasites. Perhaps it aids health and general body 
comfort. While there is some chance of infection from contaminated 
soil, the advantages of cleanliness outweigh it. In dry soil exposed to 
the sun, especially in arid regions where dust bathing is most common, 
soil-borne infections seem few, probably because of the sterilizing 
effects of full and sustained sunlight. 

Many birds use water for bathing, such as the American Robin, 
American Goldfinch, and Tufted Titmouse. Water birds, Shorebirds, 
and Passerine birds, as a rule, bathe in water (if they bathe at all). But 


exceptions occur as in the Vesper Sparrow that uses dust or in many 
birds of the arid regions. The House Sparrow, for example, has been 
observed to bathe in water as well as in dust. 


BEISTER, H. K., and L. H. SCHWARTZ (eds.), Diseases of Poultry. Ames* Iowa: Iowa 

State College Press, 1948. 
BOYD, ELIZABETH M., "The External Parasites of Birds: A Review," Wilson Bulletin, 

63 (1951): 363-369. 
CHANDLER, A. C., Introduction to Parasitology. New York: John Wiley & Sons, 

Inc., 1949. 

HERMS, W. B., Medical Entomology. New York: The Macmillan Co., 1950. 
HYMAN, LIBBIE HENRIETTA, The Invertebrates, 3 vols. New York: McGraw-Hill 

Book Co., Inc., 1951. 
*RoTiiscim,n, MERIAM, and THKRESA CLAY, Fleas, Flukes, and Cuckoos. London: 

Collins, 1950. 
*STORER, TRACY I., General Zoology. New York: McGraw-Hill Book Co., Inc., 



Rise of Bird Protection 

For a long time in his history man killed birds, if he killed them at 
all, just as did the wild predator: when he chose, when he could, and 
as he pleased. But always he killed within the limits set by the crudity 
of his weapons bare hands, clubs, snares, arrows which alone guar- 
anteed protection to birds except for nesting colonies that he might 
raid unaided by weapons. Just as the wild predator is unable to make 
its prey extinct in their respective native ranges, primitive man seem- 
ingly was not able to do so either (except possibly for Maoris preying 
upon the Moas). 


Early Bird Protection. The large and colorful birds no doubt first 
attracted man's interest (Figs. 22-1, 22-2) for food or for feathers, 
and in a sense they consequently received his first protection. Restric- 
tions of a protective nature may well have been class distinctions, 
others perhaps religious taboos (Leopold, 1933). Because only one 
who had earned a coup might wear the emblematic Eagle feather, by 
that token, the Eagle received a measure of "protection" from the 
Indian. Birds sought, like the Quetzal, for adornment of a chieftain's 
costume only, received more "protection" from primitive hunters than 
those sought by all Indians. But we can rightly hold that the bird not 
sought at all was the best protected. 

In early historic times, Kublai Khan protected "large birds" from 
March to October; I Icnry VII of England prohibited I Icrons from 
being taken except by Hawk or longbow; and James I decreed that 
"hail shot in hand guns" might not be discharged within 600 paces 
of a Heronry (Leopold, 1933). Henry VIII protected Waterfowl 
during the breeding season and James I added Pheasants and Part- 




ridges. But Henry VIII also put bounties on Crows, Choughs, and 
Rooks, which bounties Elizabeth I extended to the Magpie, Cor- 
morant, Kite, and many other birds. 

Until the coming of the shotgun and rifle, little universal protec- 
tion was needed save for control against egging, hawking, nesting, 
snaring, or nest destruction. With the coming of gunpowder came 
the destruction of birds on a large scale and consequently greater 

Fig. 22 2. Bird designs in totem art of British Columbia Indians would 
seem to testify to an esthetic response to bird life, (a) Raven, (b) Red- 
shafted Flicker, (c) Eagle, and (d) Thunder-bird, (a, b, c after Alice 
Ravenhill, An Outline of Arts and Crafts of the Indian Tribes of British 
Columbia. Occasional Papers of the British Columbia Provincial Museum, 
No. 5, 1944; d from a totem pole.) 

need for protection. In addition, there arose the attitude that birds 
which made good targets took something of value from the hunter 
or the farmer. Even today, the uninformed or willful finds this reason 
enough for the destruction of birds and other wildlife by shooting, 
trapping, poisoning, and other means. 

Later Bird Protection. In England, the first blanket protection of 
songbirds, as distinguished from game birds, hunted birds, or preda- 
tory birds, seems not to have been legislated until a Parliamentary 
enactment of 1831 (Leopold, 1933). In the New World, the first 
regulation for game was enacted by the New Netherlands colony 



in 1629. The first songbird protection was that enacted by Massachu- 
setts in 1818 which prohibited killing Robins between March 1 and 
July 4. The first blanket protection for songbirds appears in 1851 
(Connecticut, New Jersey), and we may presume that little was 
needed or considered necessary before that time. By 1883, nineteen 
of the thirty-nine states had laws protecting nongame birds. All states 
of the United States and all provinces of Canada have since bnacted 
blanket laws protecting songbirds, using the "Audubon model law" as 
the basis for legislation. 

Fig. 22*3. That the difference in voice between the American and 
European Cuckoos formed the basis for cartoon art speaks highly of 
interest in birds for their own sake. (Hy permission \rorn the cartoon strip 
Pogo, by Walt Kelly. Copyright, 1952, Walt Kelly, Post-Hall Syndicate, 
Inc., New York.) 

In the nineteenth century, protection of nongame birds arose in 
part as a desire to protect them for themselves and in part to protect 
them as protectors of crops. In the early part of the twentieth cen- 
tury, protection became so universal in most of the English-speaking 
lands that songbirds are now protected almost wholly for themselves. 
Thus protection has become a tradition, the strongest form of control 
yet devised in a democratic society, and the importance of which is 
being continually indicated (Fig. 22-3). 

Protection of Larger Birds. Protection of the larger and more 
spectacular birds, past and present, has lagged far behind that of song- 
birds. This is especially true of Hawks, Owls, and other birds alleged 
by some to be harmful. The fundamental motive, the desire to shoot 
something, still dominates treatment of predators and most large birds. 
When that motive is taken away, the willingness and zeal hunters and 
others use in killing predators should largely disappear and protec- 
tion should be substantially as good as it was before gunpowder was 

The Kite, at one time perhaps the most familiar predatory bird in 
England, long ago became one of the rarest (Newton, 1893-1896). 


The birds commonly rode the air currents over the city of London, 
from which habit came the term kite as applied to a child's toy in the 
air. Other Hawks and Owls have likewise declined in numbers and 
become scarce. In America, the larger birds have declined in much 
the same way, particularly so with the marked increase of hunters in 
the twentieth century and with the use of breech-loading shotguns in 
place of the old muzzle loader. As late as 1900, when there were fewer 
hunters (and more game), the number of Hawks clearly was still 
relatively high. Egg collecting as a hobby still had many advocates 
who seemed quite able to find Hawk nests in numbers in a day of 
search with horse and buggy. 

The claim that blanket killing of predators will increase game has 
been demonstrated to be false. But many states and provinces still 
make exceptions in the nongamc bird laws to remove protection from 
Hawks and Owls or from some, such as the Horned Owl, Cooper 
Hawk, and Goshawk. Because few hunters know one game bird from 
another, let alone the unprotected I lawks and Owls, all large birds 
are shot. To grant protection and to plug the loophole by which so 
many birds are destroyed, laws need readjustment. A model Hawk 
and Owl law providing that a landowner may kill one in the act of 
destroying his property has proved good. This protects the birds and 
the landowners (when need be) but removes the opportunity for 
hunters to make a game of shooting predators. 

Ownership of Birds. The ancient and legal designation of wild 
animals, ferae naturae, means literally "animals of nature." Others 
are domestic animals. In the United States and Canada, animals ferae 
naturae, like other things of an ownerless nature (so far as they arc 
capable of being owned), belong to the people. Because the state or 
province is the medium through which the people exercise the col- 
lective ownership, it is commonly said that the birds belong to the 
state or to the province.* In other countries, the custom may be dif- 
ferent. A common one is for game birds to belong to the landowner 
and nongame birds to belong to the state. 

Migratory Bird Treaties. The United States and Canada entered 
into a treaty, proclaimed December 8, 1916, for protection of birds 
that migrate between the United States and Canada. A similar treaty 
with the United Mexican States was proclaimed March 15, 1937. 
These two treaties mark an important event in the history of dealings 
between nations, for they dealt primarily with the protection of birds 
that the respective people might enjoy them. 

* A discussion of this and related subjects of state, provincial, and federal controls 
in the United States and Canada will be found in Wing (1951). 


Because songbirds already were well protected against hunters by 
their traditional status as nongame birds, the treaties served chiefly to 
curb hunters in some of their excess shooting of migratory game birds 
and many larger nongame ones. The inclusion of song and insectiv- 
orous birds made the coverage comprehensive and particularly served 
to bring the very great power of the bird-lovers and farmers to^the aid 
of game protection. This fact is clearly shown in the language of 
the treaty and also in the United States Supreme Court decision * 
sustaining the original Migratory Bird Treaty which gave the treaty 
effect, f 

Interest in Nature. Interest in nature as a form of enjoyment is 
a characteristic of the British, Teutonic, and Scandinavian cultures. 
It is less apparent among Latin, Slavic, Oriental, and other cultures. 
The underlying motives for this would surely be a fruitful source of 
philosophical inquiry. Since the whole subject of bird study is largely 
one of interest in birds because participants like birds and the out- 
doors, a long discussion of it seems unnecessary here. 

Many organizations for bird protection and bird study have 
flourished from time to time. Many have been in operation for a great 
many years. The number of local and regional bird clubs in the 
United States and Canada probably exceeds a thousand. The leading 
four and their dates of organization are: 

American Ornithologists' Union, 1883 
Wilson Ornithological Society, 1888 
Cooper Ornithological Society, 1893 
National Audubon Society, 1901-1902. 

Use of Bird Plumage. Nothing portrays so completely the mili- 
tant spirit of bird protection as the battle of bird lovers with the 
millinery industry over use of feathers and sometimes dead bodies of 
birds on women's hats. J 

Because America was a major disposal point for the booty of plume 
hunters, state laws and especially the Tariff Act of 1913 prohibiting 
importation, sale, and shipment of wild bird feathers largely ended 

* Missouri v. Holland, 252 U. S. 416. It should be noted, however, that the cor- 
rectness of this decision has been seriously questioned on the grounds that it estab- 
lishes an unconstitutional principle that a treaty made under the Constitution may be 
invoked as authority for federal action not sanctioned by the Constitution itself. In 
other words, "a treaty for which the Constitution is the authority overrides the 
Constitution itself." 

t40 Stat. 755 (1918); 48 Stat. 1555 (1936). 

t Magazines active in the campaign (e.g., Bird-Lore, Ladies Home Journal, Sat- 
urday Evening Post, etc.) contain many pictures of the plumage trade and stories of 
the bird lovers' battle. Files of these magazines can be consulted in most larger 


the trade. It is reported that within 1 days after President Woodrow 
Wilson signed the 1913 Tariff Act, the London market declined 
(Henderson, 1927). It seems difficult to comprehend the quantities 
of birds killed. Between 1900 and 1908, twenty to thirty camps in 
Oregon were killing Grebes for feathers. In Venezuela, 1,500,000 
Egrets were killed for plumes in 1898, and 400,000 Hummingbird 
skins were shipped to a single London dealer from the West Indies 
in one year. One London sale auctioned off 400,000 bird skins from 
America along with 350,000 from India. 

That vigilance must be constant is shown in the raid of Japanese 
poachers to the Laysan Island bird sanctuary in 1909; they killed 
more than 200,000 birds, mostly Albatrosses (Dill and Bryan, 1912). 
More than 300,000 had been killed on Lisiansky Island (in 1905) 
before the slaughter ended. The need for vigilance in another direc- 
tion was shown in the late 1930's. The Tariff Act of 1913 permitted 
the sale of feathers imported prior to its passage. A clause inserted 
in 1922 permitted importation of feathers accompanied by an affi- 
davit that they were for the manufacture of fish flies. Through these 
two loopholes, unscrupulous importers poured thousands upon thou- 
sands of feathers during a revitalized style of wearing feathers (Pough, 
1940). No feather of any wild bird has any appeal to trout or other 
fish over those of the barnyard Chicken; any assumed lure exists in 
the imagination of the fisherman.* Probably artificial flies made from 
some of the spectacular synthetic fibers and fabrics would do just as 
well and perhaps better. 

Protection of Bird Colonies and Island Bird Life. Islands in 
the seas, especially remote from land, often develop curious and in- 
teresting bird faunas. Extraordinary tameness in dealing with man 
often characterizes island birds and must be credited to a long history, 
perhaps many millions of years, without contact with ground mam- 
mals (Fig. 22-4). The Terns of South Trinidad in 1913 alighted on 
the heads to peer into the faces of men in whaleboats. Albatrosses, 
the famed Gooney birds, seem not afraid of anything, not even the 
great caterpillar tractors, autos, or people on Midway Island. Hawks 
on Galapagos Islands allowed themselves to be touched, and Fly- 
catchers tried to pull hair from visitor's heads for nest material (Car- 
son, 1950). Rats coming ashore from a wrecked steamer on Lord 
Howe Island near Australia in 1918 nearly exterminated the bird life. 
Rats cause about three-quarters of the Tern losses on Cape Cod 
(Austin, 1948). Many islands of the South Atlantic have suffered 
terrible catastrophes to their bird and plant life from the followers 

* "Wild Birds and Fly-fishing: Is America Big Enough for Both?" Circular 47, 
National Audubon Society. 



of man rats, goats, and pigs (Murphy, 1936). The Hawaiian Islands 
are truly a polyglot of alien birds introduced earlier by a misguided 
society for acclimatization of birds and today by equally misunder- 
standing sugar and pineapple interests (Fisher, 1948). Birds from 
many parts of the world now are found where once throve a unique 
and wonderful avifauna. 

C >* ; V l ' * I 

hig. 22 '4. This group of Lwperor L'eugums tobogganing leisurely 
along on the snow changed their course and came up to the photographer 
for a curious inspection. It demonstrates the tameness of birds having 
little contact with man or predatory land animals. (Photographed at the 
mouth of the Bay of Whales by the U. S. Antarctic Expedition of 1939- 
1941. U. S. National Archives.) 

A complete list of the birds that have become extinct or extirpated 
on islands would be appalling. Many were destroyed by rats and 
pigs or man's destruction of the habitat. Many have been reduced 
by military installations, airplanes, airports, and dogs. Whether the 
rats on Midway or other islands could be exterminated is not clear, 
but it would be an easy matter for dogs (and cats) to be prohibited 
from being taken to such islands. 


How many birds have become extinct since the Industrial Revolu- 
tion is a question that cannot be answered with certainty. Sixty or 
more island forms have become extinct at the hands of man through- 
out the world; fewer have become extinct on continental land. 


Nine forms of continental American birds have become extinct 
since the English settlements in America, with probable date of ex- 
tinction as follows: 

Great Auk 1853 Carolina Paraquct 1904 

Pallas Cormorant 1852 Louisiana Paraquet 1904 (?) 

Labrador Duck 1878 Eskimo Curlew 1930 (? ) 

Heath Hen 1932 Townscnd Bunting 1832 

Passenger Pigeon .... 1898 (1914) 

Great Auk. The Great Auk was a marine diving bird about the 
size of an ordinary barnyard Goose. It lived along the North Atlantic 
coasts where it nested along the shores of Norway, Orkney Islands, 
Faroes, Newfoundland, Labrador, Iceland, and Greenland. In winter 
it moved south to Maine and Massachusetts (occasionally farther) 
in the New World, and to Denmark, France, and northern Spain 
in the Old. The bird was unable to fly; its wings were merely flip- 
pers, from which it sometimes gained the name of "Penguin," a cor- 
ruption of "pin-wing." Like most sea birds the Auks had a heavy 
layer of fat, which proved to be a source of oil and their undoing. 
For this oil and as food also, the birds were relentlessly pursued. I Tow 
many birds the oil gatherers killed is not known, but at the height 
of the oil business, the numbers doubtless ran into scores of thousands 
of Auks a year. The birds were usually killed with clubs on land 
where they were helpless or clubbed in the water where they showed 
little fear of man. Concentrations of birds at the nesting grounds par- 
ticularly drew the despoilers. 

The last bird found on the great nesting ground of Funk Island 
appeared in 1840. One was reported for Iceland in 1844; the last 
living bird was seen in 1852, but a dead bird was picked up in Trinity 
Bay, Ireland, in 1853. Though people have written of the "unknown 
cause" of its extinction, killing by man is the correct one (Newton, 
18931896). Only about eighty mounted and unmounted specimens, 
four skeletons, and a few eggs are known to be preserved. The 
American Ornithologists' Union publishes its quarterly journal under 
the title of The Auk in memory of the extinct Great Auk. 

Pallas Cormorant. The Pallas Cormorant was discovered in the 
Bering Islands off Alaska in 1741 by the expedition of Vitus Bering. 
He reported them there in some abundance, but the bird became ex- 
tinct by 1852. The exact sequence is unknown, but the birds were 
killed ruthlessly for food. 

Labrador Duck. The breeding grounds of the Labrador Duck 
remain unknown, and the bird became extinct without its nest having 
been found. Probably it nested on the Ungava coast, the western 



part of Labrador. It wintered along the Atlantic Coast from Nova 
Scotia to New Jersey and perhaps to Chesapeake Bay, the heart of the 
market hunting and shooting country. The reason for its extinction 
is excessive shooting. The last authentic record of the Labrador 
Duck was December 12, 1878. 

Heath Hen. From Maine to Virginia and perhaps west into Penn- 
sylvania and Ohio lived an eastern form of the Prairie Chicken known 
as the Heath Hen. It obtained its name because the Pilgrims found 

Fig. 22-5. The last Heath Hen. Male photographed on Martha's Vine- 
yard, Massachusetts, March 31, 1930. (Photograph by A. O. Gross.) 

it living rather abundantly upon the coastal areas and brush regions of 
the East, especially Cape Cod, Long Island, and similar areas of shore 
and scrub pine. 

By 1840 they were gone from the mainland of Massachusetts and 
Connecticut, though persisting longer on Long Island and the bar- 
rens of New Jersey and eastern Pennsylvania. By 1870 they had 
become restricted entirely to the small island of Martha's Vineyard 
off the southeast coast of Massachusetts. In 1890, there were some 
200 birds on the island, but the number dwindled to about 100 in 
1896 and to 50 in 1908, when a reservation was established for them. 
Intensive effort of Massachusetts people increased the number to 
about 2,000 in 1915, when they were well distributed over the island. 


But an unfortunate sequence of events followed, which because un- 
controlled spelled the doom of the birds. Fire swept the island on 
May 2, 1916, burning some twenty square miles of area and destroy- 
ing many nests and birds. The following winter was exceptionally 
severe, so that in 1917, only 150 birds were found. Most are believed 
to have been males, which probably reflected the heavy loss of incu- 
bating females in the spring fire of 1916. In 1920, domestic Turkeys 
brought to the island apparently introduced blackhead and other 
diseases. By 1927, the number of Heath Hens declined to eleven 
males and two females. In the fall of 1928, only two males were 
found and on December 8 but one male remained. This bird per- 
sisted until last seen February 9, 1932 (Fig. 22-5). 

Passenger Pigeon. Passenger Pigeons formerly nested from cen- 
tral Mackenzie, southeast to central Quebec, and Nova Scotia, south 
to Kansas, Mississippi, Kentucky, and Pennsylvania. They wintered 
from Arkansas and North Carolina south to Texas and Florida. The 
general range of the bird appears to have covered most of the area 
between the Rockies and the Atlantic. 

The Passenger Pigeon migrated and nested in flocks so large as to 
seem almost unbelievable (page 256). They nested in great colonies, 
where each female laid but a single egg. People killed them at all 
seasons of the year, on their nesting grounds, in migration, and in 
winter. (A similar situation still exists in the White-winged Dove 
colonies of northern Mexico, where "sportsmen" stand at the road- 
sides and shoot great numbers of Doves returning to the nests in the 
late afternoon.) 

The first law to protect the Pigeon seems to have been enacted by 
New York in 1867; Michigan gave a small measure of protection 
in 1869; Massachusetts in 1870, and Pennsylvania in 1878. The last 
bird reported in the wild was in 1898, when some were seen at Lake 
Winnipegosis, Manitoba, April 14; July 27 at Owensboro, Kentucky; 
September 14 at Canandaigua, New York; and September 14 at De- 
troit, Michigan. Despite real interest no authentic reports have oc- 
curred since, except for one fairly reliable report of birds at Bab- 
cock, Wisconsin, in September, 1 899. The last known specimen died 
in the Cincinnati Zoo at the age of 22 years in 1914; it had been 
hatched in captivity. A monument commemorating the Passenger 
Pigeon has been erected in Wyalusing State Park, Wisconsin. 

Contrary to popular belief, the Passenger Pigeon did not "disap- 
pear overnight," but its disappearance was a progressive one extend- 
ing over a 50-year period or longer (Schorger, 1937). Because of its 
erratic habits, its absence in an area was attributed to a shift of flights 
rather than to a decline in numbers. Many causes and circumstances 


contributed to the extinction of the bird. Among them were the 
cutting of timber and settling of the land where they nested, the 
habit of laying but a single egg, nesting but once a year, destruction 
during nesting, and destruction in migration and in winter. 

Paraquets. In southeastern and central United States formerly 
lived a species of Paraquet separated into two subspecies Joiown 
respectively as Carolina and Louisiana Paraquets. (The latter, inter- 
estingly enough, was described as new to science only after extinc- 
tion.) These birds formerly lived north from Florida to southern 
Virginia and west to Indiana, Oklahoma, and Louisiana. The only 
member of the Parrot family resident in eastern United States, the 
birds were reported in large flocks by observers in the early 1800's. 
The last one in Illinois was seen in 1861; none has been seen in 
Louisiana, Mississippi, Kentucky, or Tennessee since 1880. The last 
one disappeared from Kansas and Missouri in 1904. An obscure re- 
port placed them in Florida as late as 1914 and possibly 1920. The 
Paraquets became extinct because of destruction of their habitat and 
shooting the birds for food and as destroyers of crops. Their Parrot 
nature of returning and hovering over a fallen comrade made it pos- 
sible for a gunner to destroy a whole flock. 

Eskimo Curlew. The Eskimo Curlew formerly nested on the 
Barren Grounds and migrated in autumn southeastward to the coast 
region extending from Labrador to New Jersey. From there it 
crossed the western Atlantic to South America and wintered in the 
Pampas country. Audubon visited Labrador in 1823 and gives us this 
description of their numbers: "The accounts given of these birds bor- 
der on the miraculous. They arrive in such numbers to remind me of 
the Passenger Pigeon." In spring they migrated northward through 
the continental interior. The great flocks were decimated at all 
seasons of the year (save possibly on some of the nesting grounds), 
from Labrador to Argentina and back through the interior of North 
America. By 1890 only a few scattered flocks were reported any- 
where. The last individuals reported arc about as follows: Ohio, 
1878; Michigan, 1883; Indiana, 1890; Wisconsin, 1912; Argentina, 
1925; and Nebraska, 1926. Unconfirmed but evidently reliable re- 
ports have placed Eskimo Curlews in Labrador as late as about 1930 
and probably a few scattered elsewhere during the following years. 

Townsend Bunting. The Townsend Bunting, a close relative of 
the Dickcissel, has puzzled taxonomists, who have been unable to 
ascribe its characters to hybridization. This leads to the suggestion 
that it may have been an ancient species that became extinct (1832) 
before a second specimen was obtained. 


Island Birds. Birds reported extinct upon islands adjacent to 
North America are: 

Black-capped Petrel 1920 (?) 

Guadalupc Caracara 1900 (? ) 

Cuban Tri-colored Macaw J864 

Cossc Macaw 1800 

Guadeloupe Macaw ? 

The birds that appear to have become extinct on the islands of the 
world are numerous; yet we are unable to record with any certainty 
the extinct ones on the islands near North America. 

The finding of a few living individuals of supposed extinct birds 
gives some hope from time to time that others considered extinct may 
still be found. The Cahow, a Bermuda Petrel supposed to have been 
extinct, has been found in very small numbers at Bermuda. A few 
individuals of Notornis, a flightless member of the Coot and Rail 
family have been found alive in some remote meadows of New Zea- 
land after long having been considered extinct. But the possibilities 
of finding many supposedly extinct birds extant are greater than the 

When the Dutch discovered the island of Mauritius toward the 
end of the sixteenth century, they found a very large, flightless mem- 
ber of the Pigeon family, the Dodo. It was easily killed by men but 
especially by hogs gone wild. The bird was termed stupid because it 
had no fear of man, and the word do do still means a stupid one. An- 
other extinct Dodo lived on the island of Reunion to the south of 
Mauritius, where also lived a now-extinct Starling. On the island of 
Rodriguez, to the east of Mauritius, lived many interesting birds now 
extinct: a Dodo (called the Solitaire), Owl, Parrot, Dove, Heron, 
Rail, and a flightless Rail. Clearly, the protection of island bird life 
is probably the most important single need in international protec- 
tion of birds. Continental forms with their large populations and 
large land areas have far fewer difficulties in surviving. 


It would not be possible to list all the rare or threatened birds of 
the world, for rarity often means only that the region wherein the 
species is rare lies at the edge of the range. Within the United 
States and Canada, however, are a number of birds that should be 
considered as rare or threatened species: 

Great White Heron Spruce Grouse 

Glossy Ibis Attwater Prairie Chicken 

Ross Goose Sage Grouse 


Trumpeter Swan Masked Bob-white 

California Condor Whooping Crane 

White-tailed Kite Sandhill Crane 

Swallow-tailed Kite Roseate Tern 

Mississippi Kite Aleutian Tern 

Everglade Kite White-crowned Pigeon 

Red-bellied Hawk White-winged Dove 

Short-tailed Hawk Red-billed Pigeon 

Zone-tailed Hawk Florida Burrowing Owl 

Sennett White-tailed Hawk Thick-billed Parrot 

Gray Hawk Ivory-billed Woodpecker * 

Harris Hawk Colinia Warbler 

Mexican Black Hawk Golden-cheeked Warbler 

Audubon Caracara Kirtland Warbler 

Peregrine Falcon Sutton Warbler (probably a 
Aploniado Falcon hybrid) 

Franklin Grouse Cape Sable Seaside Sparrow 

Efforts to Protect Rare and Threatened Species. While credit 
must go to the United States and Canada for their efforts to protect 
some of the rarer and threatened birds, the blame for the birds need- 
ing extra effort must go with the praise. The most promising develop- 
ment is not an official one at all but the simple recognition by many 
people (particularly those connected with "tourist industries") that 
the presence of a unique bird or other animal can be a source of civic 
pride and even attraction. Thus, guides along the Florida Keys point 
out the Great White Herons, Roseate Spoonbills, and other birds as 
attractions, which status their appearance and uniqueness rightly 
confer upon them. The establishing of a bird as a living symbol, such 
as that of the Bald Fagle as the American national emblem, gives it 
protection (except in Alaska). A special form of "living public 
monuments" has been proposed as a protective status, just as historical 
sites are protected as historical monuments. 

Great White Heron. The Great White Heron, a pure white bird 
about the size of the Great Blue Heron, lives in the Florida Keys. 
It has never been an abundant bird in comparison with other Herons, 
and has been reduced in numbers by man's pre-empting its habitat, 
by malicious shooting (simply because it was a large and an easy 
target), and killing of young in the nests by fishermen. Some have 
been destroyed by hurricanes, especially important destructive agents 
when the birds are in low numbers. Prime credit for its protection 
should go to the National Audubon Society and also to the Florida 
Game Department. The establishment of a Federal Wildlife Refuge 
makes certain that still more protection will be afforded the species. 

* Probably extinct. 


Roseate Spoonbill. The Roseate Spoonbill formerly inhabited 
much of the Gulf coast region from Texas to southern Florida, south- 
ward to Argentina and Chile. The birds were forced out of the 
Gulf coast, largely by hunters, but are staging a comeback under 
increased protection (Allen, 1947). 

Ross Goose. The Ross Goose is a small white goose about the 
size of a Mallard Duck. It breeds along the Perry River west of 
Hudson Bay in an area so small that its nest was unknown until 
1938. There are probably fewer than 6,000 of the birds as a total 
population. The birds migrate south to winter in the Sacramento and 
San Joaquin valleys of California. 

Trumpeter Swan. The original breeding range of the Trumpeter 
Swan extended from Alaska across to James Bay and south to Indiana, 
Missouri, Wyoming, and Montana. The birds have been reduced in 
number by shooting and settlement of the range. Remnant popula- 
tions of the birds live in Yellowstone Park, Red Rock Lakes of Mon- 
tana (near the Yellowstone), and in British Columbia. The increase 
in numbers has made it possible to transfer breeding stock to other 

California Condor. The California Condor lived along the Pacific 
Coast region from Lower California north to at least the lower Colum- 
bia River. By 1930, the birds had dwindled to a few in the Coast 
Range of southern California. Because the Condor was a large and 
clumsy bird, it fell easy victim to malicious shooting. Its habit of 
perching during early morning and cloudy weather made it an easy 
target. Many died from eating carcasses put out as poisoned bait by 
ranchers and the Biological Survey (more recently Fish and Wildlife 
Service), a greater danger to all birds now than ever before because 
poison campaigns are more widespread and efficient. 

The birds lay but a single egg on bare ledges, and nest every other 
year, perhaps at longer intervals. Incubation takes a month and the 
young are covered with down in another 2 months. They become 
fully feathered at about 5 months and are able to fly somewhat later. 
Five years seem necessary to reach adulthood (Koford, 1953), when 
the birds weigh about 20 pounds and have a wing spread of 9 to 10 
feet. They are a relatively ancient species, more widespread in the 
Pleistocene (see Fig. 9-5). 

Attempts to protect the Condor from extinction have centered 
around protection from disturbance and especially people with guns. 
The Forest Service has closed the remnant area to people. But the 
supposed presence of extractable natural resources has caused pres- 
sure for commercial entrance to the area, which probably would spell 


the doom of the species. Continuance of poison campaigns likewise 
is a danger that the Condor may not be able to overcome. 

Kites. The Kites all are gentle birds, never destructive of any real 
or assumed interest of man. The White-tailed Kite formerly ranged 
from California to Florida and south to Lower California and Guate- 
mala. It has never been abundant in the United vStates, and probably 
is found now in but a few California locations. The Swallow-tailed 
Kite formerly ranged as far north as Minnesota, but is found now 
only along the Gulf coast area. The Mississippi Kite formerly inhab- 
ited the region from southern Illinois and South Carolina south to 
Florida and Texas. It is found today only in the Mississippi Valley 
and adjacent country. 

The Everglade Kite ranged formerly from northern Florida south 
to South America. Fire and drainage in the Everglades have reduced 
the number of birds, along with the ever-present shooting and dis- 
turbing of large birds by people everywhere. 

Unique among Hawks and Falcons in fact, unique among birds 
is its method of feeding upon snails of the genus AmpullaYius* The 
Kite darts and skims about over the pools and their shores, usually 
during late afternoon when snails move about most and crawl from 
the water onto the stems of vegetation. It grasps a snail in one foot 
and retires to a perch, still holding the snail gently in one foot. The 
bird makes no attempt to obtain its food by force; it waits for the 
voluntary extension of the animal from the shell. When this happens, 
the Kite quickly pierces the snail behind the operculum, always in 
the same place, which is evidently a nerve plexus. The Kite then sits 
and waits again, with the snail spiked on its bill, from which it stands 
out like a bump as large as the bird's head. Gradually the muscles of 
the numbed snail relax. After two minutes, more or less, the Kite 
vigorously shakes its head and swallows the mollusk, operculum and 
all, before the empty shell has reached the ground. The fragile shell 
never is broken or abraded by the captor. The long, slender bill, 
delicate and rather flexible, serves not as a hook but as a lancet or 
poniard. It is a case of instinctive correlation as exact as that of the 
spider-paralyzing wasps. 

Audubon Caracara. The original range of the Caracara extended 
from Ari/.ona to Florida and south to South America. The numbers 
of the bird have been reduced by shooting and disturbances of settle- 
ment. As the national emblem of Mexico, the Caracara frequently is 
called "Mexican Eagle." 

* Dr. Robert Cushman Murphy generously supplied the description of Kite 
feeding from his unparalleled opportunity for observation in the Argentine pampa. 


Attwater Prairie Chicken. The original range of the Attwater 
Prairie Chicken was unique in that it was separated from the main 
range of the Prairie Chicken. It covered a rather small area on the 
Texas Gulf coast. The species has been greatly reduced in numbers 
by shooting, farming, and ranching. The increase of brush on many 
ranches as the result of overgrazing and destruction of grasses and 
forbes seem to have hurt the Chicken also. Little has been done to 
try to restore the birds, principally because people in the region are 
not sufficiently aware of its unique character, and official bodies have 
been concerned primarily with birds in numbers sufficiently large to 
interest hunters. 

Whooping Crane. The Whooping Crane formerly bred in small 
numbers over the region from Hudson's Bay to the Mackenzie River 
south to about Nebraska. Today, the only remnant winters on the 
Gulf coast of Texas (chiefly in Aransas County) and breeds in 
northern Canada. The nesting place of the remnant or part of it was 
found in the summer of 1952 to be near Great Slave Lake. The 
Cranes migrate south across the plains country. They seem to stop 
in the Platte River Valley in Nebraska. Some birds are shot each 
year by hunters. Because the birds number fewer than a score (page 
261), any shooting is a disaster to the species. The Aransas National 
Wildlife Refuge has been established for the Cranes, though in its 
actual administration, oil, cattle, and deer have a higher "priority" 
than the Cranes. 

Ivory-billed Woodpecker. The Ivory-billed Woodpecker (pos- 
sibly extinct) formerly inhabited the southern swamps from the Caro- 
linas to Louisiana and East Texas, north to Oklahoma, Kentucky, and 
Missouri river bottoms. This Woodpecker, the largest of all Ameri- 
can Woodpeckers, is reported to have a bill especially adapted for 
scaling bark from recently dead cypress trees to get borers under- 
neath. Because its supply of food is so restricted, only mature trees 
furnished it in quantity. Hence, the species suffered from the lumber- 
ing of cypress timber. 


Winter Feeding. Few things testify so frequently to the compas- 
sion of man toward birds as winter feeding. It should be borne in mind 
always that birds must find something to eat and must withstand the 
elements by their own efforts at all times. When a human looks out 
of the window from its safe side during a storm winter, spring, fall, 
or summer he should bear in mind the inescapable fact that birds 



must live in all types of weather. No matter how severe the storm 
or how bad the weather, a bird still must look for food. The length 
of time that it can "hole up" is limited. 

Bird-feeding trays adorn many home grounds, along with bird 
boxes of summer. Insectivorous birds require animal foods, which 
customarily are fed to them as suet or chopped meat. Seed-caters 
will eat little except small grains, cracked corn, and chopped nuts. 
But Nuthatches, Chickadees, and their companions will eat a variety 
of foods seeds, grains, or suet. Birds that feed upon berries and 
fruits, like the American Robin, will eat raisins, apples, and other 
similar foods. 

Foods commonly used in winter feeding and some examples of 
birds using them follow: 

Suet: Woodpeckers, Jays, Chicadees, Nuthatches 

Small grains: Jays, Thrashers, Mockingbirds, Cardinals, Finches, Juncos 

Sunflower seeds: Chickadees, Nuthatches, Cardinals, Grosbeaks, Finches 

Raisins: Mockingbirds, Thrushes, Waxwings 

Apples: Waxwings, American Robins, Cardinals 

Trays for feeding may be a window shelf, a tree shelf, a trolley 
shelf, or a feeding shelter (Fig. 22 6) . Some birds may be fed by scat- 
tering grain upon the ground. Snow may cover scattered grain, and 
ground-feeding birds need some assurance of protection from dogs 

Fig. 22 6. Many kinds of bird houses can be made, that on the left in 
mass production by use of six machine-cut pieces. The top way be lifted 
off for cleaning and disinfecting. Sometimes cats and squirrels must be 
kept from a feeding station, nest box, or tree; a guard such as that in the 
center may be effective. A revolving feeding shelter as shown at the right 
will prevent snow, rain, or wind from covering or blowing out the food. 
If House Sparrows are a nuisance, the front may be made into a hinged 
shelf held by rubber bands or weak springs. House Sparrows tend to avoid 
such a shelf, though native birds do not seem to mind it. 


and cats. Feeders often are placed on posts or platforms or covered 
by a brush shelter loose enough to protect an escaping bird. 

Summer Feeding. Summer feeding differs little from winter feed- 
ing except that animal foods like suet are less sought after than in 
winter and are also more likely to spoil. In some gardens, more birds 
come to feeding stations in summer when adults seek food for young 
or when the young are about than at other seasons. Water attracts 
birds in hot weather especially but at other times of the year also and 
can be provided as a bird bath, fountain, or dish. In winter, a dish 
warmed by an electric light bulb below it will be kept from freezing. 
Some birds seem more attracted to water than others. In the North, 
Bohemian Waxwings come to water in winter possibly more than 
any other species. American Robins, Grackles, Song Sparrows, and 
Mockingbirds are regularly attracted in warm weather and sometimes 
in cold weather also. 

Planting for Birds.* The shrubs and trees planted for bird foods 
vary throughout the continent, so that few general suggestions can be 
made. The leading shrub or tree fruits are mulberry, blackberry, rose, 
mountain ash, cherry, elderberry, service berry (Awelawchier spp.), 
sumac, holly (Ilex spp.), grape, dogwood, and Viburmiw. In some 
areas, plants like thorns (Crataegus spp.), red cedar (Jumperus), oak 
(spp.), bayberry, and others are especially important, but a large 
number of native woody plants are usable (Van Dersal, 1938). 

Small weed patches or food patches of grains are eagerly sought 
by many migrating and wintering birds. Small grains of several vari- 
eties can be planted, alone or in mixture. Among those used are corn, 
fcterita, milo-maize, millet, soybean, buckwheat, sunflower, and 
others (Baker, 1941; Wing, 1951). 

Provisions for Nesting Sites. For nesting sites, thick shrubs hav- 
ing numerous forks and branches are preferred by many common 
birds. Vines trailed over bushes and trees may be useful. A wide va- 
riety of trees and shrubs may be used, depending upon the region and 
site. Considerable success has been had by dwarfing, cutting, twisting, 
and tying branches for shelter. 

Nest boxes have long been provided by man; even tribesmen in 
many parts of the world have put out hollow gourds for birds. Nest 
boxes may be of many sizes and shapes and of many materials. The 
more natural-looking they are perhaps the better, though how much 

* The Audubon Guide to Attracting Birds (Baker, 1941) should be consulted 
for detailed information on attracting by planting, feeding, providing nest boxes, 
providing water, and providing sanctuary. Practice of Wildlife Conservation (Wing, 
1951) has much information on planting and feeding along with other management 
practices for birds. 



preference birds show for various types of houses still remains a 
question. A convenient style that may be produced in mass is shown 
in Fig. 22-6. Boxes of wood and other material of low heat conduc- 
tivity are preferable to metal ones, which may cause overheating. 
Most common birds tend to prefer houses in full or nearly full sun 
for part of the day at least. Some recommended dimensions arc 
shown in Table 22-1. 

Table 22*1 
Sizes of Nest Boxes and Recommended Height Above Ground 






















American Sparrow Falcon.... 

, . 8x8 





Barn Owl 






Saw-whet Owl 

, . 6\6 



2 1 A 


Screech Owl 

, . Hx 





Hairy Woodpecker 

, . 6x6 



l'/ 2 


Downy Woodpecker 

.. 4x4 





Red-headed Woodpecker 

, . 6x6 





Golden-fronted Woodpecker. 

. . 6x6 






.. 7x7 





Crested Flycatcher 

.. 6x6 






. 6x6 





Violet-green Swallow 

. . 5x5 





Tree Swallow 

,. 5x5 





Barn Swallow 






Purple Alartin , 

. . 6 x 6 






.. 4x4 






,. 4x4 





Nuthatches , 

. . 4x4 





House Wren 

,. 4x4 





Bewick Wren 

.. 4x4 





Carolina Wren 

,. 4x4 






.. 5x5 





American Robin 

. . 6x8 





House Finch 

, . 6x6 





Song Sparrow 






* One or more sides open, 
t All sides open. 

Source: By permission from Practice of Wildlife Conservation, by Leonard W. 
Wing, p. 303. Copyright, 1951, John Wiley & Sons, Inc., New York. 

Birds that nest on cliffs, ledges, and the like, sometimes take to 
man-made structures (Fig. 22-7). The Chimney Swift nests and 
roosts in chimneys, and at least one interested ornithologist built an 
artificial structure for them. Horizontal wooden strips nailed across 
the side of a barn under the eaves increased the number of Cliif Swal- 
lows on a Wisconsin barn (Buss, 1942). Additional aid seemed de- 



Fig. 22 7. An uneven spot in the masonry of a culvert provides support 
for the Phoebe nest. Putting a slot in the fonn for the concrete would 
leave a usable ledge for the Phoebes; one in each new culvert would greatly 
increase the Phoebe population. (By permission from Practice of Wild- 
life Conservation, by Leonard W. Wing, p. 304. Copyright, 1951, John 
Wiley & Sons, Inc., New York.) 

sirable in the form of a mud source within three-quarters of a mile, 
House Sparrow control, and removal of old nests each fall. 

Nesting material may be provided for some birds. Egrets have 
taken sticks, Gulls and Terns may use straw, and Eastern Gold- 
finches will take cotton. American Robins will carry off string or 
yarn, but the pieces should be not more than about a foot long to 
prevent tangling and subsequent tragedy. Chipping Sparrows have 
taken hair, and several birds have used moss provided for them. Pro- 
viding mud to Robins, Swallows, and others that use it in the nest is 
obviously a help. 



ALLEN, ROBERT PORTER, The Roseate Spoonbill. National Audubon Society, Re- 
search Report No. 2, 1942. 

ALLEN, ROBERT PORTER, The Whooping Crave. National Audubon Society, Research 
Report No. 3, 1952. 

*BAKER, JOHN H. (ed.), The Audition Guide to Attracting Birds. Garden City, 
N. Y.: Doubleday & Co., Inc., 1941. 

*BEARD, DAN (cd.), Fading Trails. New York: The Macmillan Co., 1942. 

*GABRIKLSON, IRA N., Wildlife Conservation. New York: The Macmillan Co., 1941. 

HENDERSON, JUNIUS, The Practical Value of Birds. New York: The Macmillan Co., 

HORN ADA Y, WILLIAM T., Our Vanishing Wildlife. New York: New York Zoological 
Society, 1913. 

KOFORD, CARL B., The California Condor. National Audubon Society, Research 
Report No. 4, 1953. 

PEARSON, T. GILBERT, "Fifty Years of Bird Protection," in Fifty Years' Progress of 
American Ornithology, 1883-1933. Lancaster, Pa.: American Ornithologists' Union, 

PEARSON, T. GILBERT, Adventures in Bird Protection. New York: Applcton-Century- 
Crofts, Inc., 1937. 

RoniscHiLD, WALTFR, Extinct Birds. London: Hutcheason & Co., 1897. 
*SCHORGER, A. W., The Passenger Pigeon: Its Natural History and Extinction. Madi- 
son, Wis.: University of Wisconsin Press, 1955. 

TANNER, JAMES T., The Ivory -billed Woodpecker. National Audubon Society, 

Research Report No. 1, 1942. 

*WING, LEONARD W., Practice of Wildlife Conservation. New York: John Wiley & 
Sons, Inc., 1951. 


Economic Relations 

of Birds 

The economic values of birds to man may be listed for brevity as 
positive, negative, or neutral. No native American bird has negative 
values outweighing its positive ones, h'vcn so persecuted a bird as the 
Crow, the Hawk, or the Owl has "beneficial" values greater than 
"harmful" ones, though hunters who find less game year by year like 
to believe otherwise. Birds do "favors" for man season in and season 
out; any "harmful" acts are of short duration, but man remembers 
them long. 

Most of the economic values commonly attributed to birds center 
around their services in eating unwanted insects or other unwanted 
things. For many years, economic ornithology was practically sy- 
nonymous with food-habits study. An imposing list of other eco- 
nomic values applicable to birds appears, nevertheless, and for con- 
venience they have been classified as follows (Wing, 1951): 


Esthetic (nature study, their presence) 

Recreation (bird watching, hunting) 

Useful products (food, wearing apparel, manufactured articles, down, 

Useful activities in the wild 

Control of plants and animals 

Distribution of plants and animals 

Scavenger service 

Health protection (control of disease carriers, such as rats and mos- 

Sources of employment (ornithologists, naturalists, teachers, protectors) 
Crop saving 
Useful activities in confinement (experimental birds, domestic birds, trained 

birds, exhibits) 





Destruction of property 

Competition with or predation upon "preferred" animals 

Distribution of unwanted plants 

Encouragement of undue trespass 


Food Habits Study. The study of bird food habits arose from 
the natural desire to know more of birds and especially the economic 
desire to know how birds influenced crops and other interests of man. 
The stomach contents have long been used for study of food habits. 
For correct procedure, however, food habits must be studied not 
only by means of foods eaten but also by observation of habits, study 
of food availability, and determination of the bird's place in the 
ecological picture. 

Fig. 23.1. The remains from several American Robin droppings. This 
illustrates the ease 'with which identification of food items can sometimes 
be made. Cutworms, grasshopper fragments, larval European elm-lea^ 
beetles, ants, various carabid elytra, entire weevils, cherry seeds, blue 
nightshade seeds, and honeysuckle seeds can be recognized. (Photograph 
by William J. Hamilton, Jr.) 


In addition to analysis of stomach contents (including sometimes 
the entire digestive tract), biologists use pellets (indigestible parts 
regurgitated by predators, insectivorous birds, and some others), 
and fecal droppings (Fig. 23-1). The hard parts of insects, seeds, 
bones, and scales usually are identifiable. Sometimes also feathers, 
hair, and soft parts can be distinguished. Even needles and leaves, 
along with parts of some other vegetation, may be identified by struc- 
ture. For this analysis of food items, the stomachs are preserved dry 
or wet usually dry for grain and seed eaters, in formalin usually for 
soft foods and animal parts. The material to be examined is spread 
out and studied under a low magnification. Suitable samples of seeds, 
bones, hair, or feathers are needed from time to time for comparison. 
Keys and published descriptions of seeds, bones, hair, and feathers 
prove useful. 

The amount of each item eaten is reported in several ways, choice 
being governed by species, the kind of food, kind of research being 
made, and, with little doubt, the preferences of different ornitholo- 
gists. The most commonly used measures are by weight, volume, fre- 
quency of occurrence, number of each item, and per cent of the 

Food Adjustments of Birds. The food habits of the bird and the 
food needs of its body have become adjusted through evolutionary 
processes. The bird has evolved a digestive system and physiological 
constitution for using foods of a narrow or wide choice, as the case 
may be. The feeding instincts and structural capabilities ordinarily 
determine a bird's choice of foods. (Availability, however, may de- 
termine what it can get.) 

The Short-eared Owl, for example, has an instinct to prey upon 
rodents, talons with which to seize them, a bill for tearing the prey, 
flight (noiseless) with which to seek food, eyes with which to see in 
the dark, and a digestive system to make the food available for the 
blood stream to distribute throughout the cells of the entire body. 
An Owl coursing over a stubble field (in the daytime if need be) will 
not be attracted by the shattered wheat kernels and weed seed, though 
a Slate-colored Junco will. A meadow mouse feeding on the wheat 
or weed seed will release a feeding (i.e., preying) reaction in a 
hungry Owl but not in a hungry Junco. The grains and seeds, on 
the other hand, will release the feeding reaction in the Junco but not 
in the Owl. A truly intricate system is the bird in relation to its food. 
Feeding goes on so regularly, however, that one rarely appreciates 
the complex actions working so smoothly together. Even this brief 
account hardly indicates its ecological nature, the step-by-step mesh- 
ing of innumerable biological cogwheels (Chapters 10, 11). 



Food Consumption. As a general rule, birds eat a greater amount 
of food pound for pound of body weight than other organisms. But 
very great differences in the available calories of various foods make 
direct comparisons difficult. While meat amounting to 1 or 2 per 
cent of the body weight might alone be enough daily food to sustain 
a moderately active man, evidently no bird could get along on so 
little. In general, the bird consumes more food because it lives a rapid 
life, has a high temperature, and has a small body. 

Table 23*1 
Daily Food in Relation to Body Weight 


Body Weight 

Daily Food 
in Per Cent of 
Body Weight 

Domestic Goose 

. . 1800 


Common Buzzard 



Pitrcon . . . . ... 



Tawny Owl 

. 442-475 





American Sparrow Falcon , 






Little Owl 



Dove . . . ... 






English Blackbird , 






Alourninff Dove 



Song Thrush 






Great Titmouse 

... 18 





British Goldfinch 



Blue Titmouse 



* Seeds and grains. 

** Dry weight of meal worms (estimated at 40 per cent of live weight). 
Data on European birds from Margaret Morse Nice, "The Biological Significance 
of Body Weight," Bird-Banding, 9(1938): 1-11. 

In the discussion of the Ber ginajin Rule earlier (page 185), men- 
tion was made of the fact that larger birds with their lower bulk-to- 
surface ratio tend to have the advantage over their smaller fellows in 
colder regions. This advantage evidently lies partly in the difference 
in relative metabolic rates (perhaps reflecting surface areas). Meta- 
bolic rate is reported to vary among experimental animals approxi- 
mately with the two-thirds power of body size (page 83). How 
this would compare with birds in the wild is not known; but one 
should expect the difference to be similar. Data on the percentage of 
food eaten relative to weight of the bird show (Table 23'1) an in- 


crease in food with decline in body size (Fig. 5-7). A bird eats more 
in cold weather when energy demands are high than in hot weather 
(page 86). Measurements of birds in Europe indicate that the dif- 
ference can be very great for different temperatures. That larger 
birds can get along on proportionately less food than smaller ones is 
also shown in Table 23*2. 

Table 23 -2 
Daily Food Consumption in Relation to Body Weight and Temperature 

Food Consumption in Per Cent 
Species Body Weight of Body Weight at 

^ (Grams) 

65 F. 48 F. 45 F. 

Masked Weaver 40 20 25 28 

Red-billed Weaver 18 28 30 13 

Source: Margaret Morse Nice, "The Biological Significance of Body Weight," 
Bird-Banding, 9(1938): 1-11. 

As would be expected, the amount of food consumed will vary 
with its nutritional value (page 256). In general, animal foods sup- 
ply more usable calories for each unit than do vegetable foods. Seeds 
and grains of various kinds are somewhat less concentrated than ani- 
mal foods, though some seeds are more concentrated than some animal 
foods. Fruits tend to be more nourishing than leafy foods such as the 
grasses eaten by grazing Geese. 

Designation of Birds by Food Habits. For convenience, food 
habits serve as the basis for grouping many common birds. Birds 
within these groups have somewhat similar habits and structures 
owing to their similarity of food choice (see Chapter 3): 

Carnivorous, meat-eaters 

Herbivorous, plant-eaters 

Frugivorous, fruit-eaters 

Omnivorous, eat both plant and animal foods 

Insectivorous, insect-eaters 

Granivorous, seed-eaters 

Piscivorous, fish-eaters 

Because more than eight thousand species of birds spend so much 
of their waking lives looking for something to eat in competition 
with each other, with others of their own kind, and with other ani- 
mal life we need not be surprised therefore that a tremendous vari- 
ation in food habits exists. In a sense, for everything edible, there 
is some bird to eat it (either first- or secondhand) but with some 
notable exceptions. Few or no birds feed primarily upon some readily 



| Animal gS| Berry [S3 Needle EZ3 Misc. plant 

Fig. 23*2. Graphical representation of the food of the adult Blue 
Grouse, a "conifer forage feeder" which obtains nearly three-quarters of 
its yearly food \roin needles. The figures represent percentages of the 
total eaten. This illustrates a commonly used method of showing food 
habits. (By permission ]row Practice of Wildlife Conservation, by 
Leonard \V. Wing, p. H4. Copyright, 1951, John Wiley & Sous, Inc., 
NCIV York.) 

and widely available vegetation. The oak, maple, and other decidu- 
ous leaves of the forest, for example, are eaten but little by birds; none 
use them as a staple food. The same applies largely to other "forage" 
of the field, brush, and forest. A few gallinaceous birds eat the 
leaves of some plants, sometimes in great quantity. Sage Grouse live 
for long periods almost exclusively upon leaves of the desert sage- 
brush (Artemisia). Geese may graze, but for the most part, birds 
leave this food supply to the rabbits, rodents, ungulates, and insects. 
Almost no mammal and but a few Grouse (Dendragapus, Caiia- 
chites, Tetrao, Falcipemiis, and sometimes Lynmis) eat so many 
conifer needles that needles form a major portion of their diet. In a 
sense, they are "conifer forage" feeders, a rarity in the entire verte- 
brate world (Fig. 23-2). Naturalists afield may wonder justly at the 
seemingly remarkable fact that among the vertebrates, only the Tree 
Grouse and two rodents have taken to conifer forage, one of the 
world's great vegetational resources, as a staple diet. 


Among birds with interesting kinds of food habits may be men- 
tioned the Everglade Kite that feeds solely or nearly so upon snails 
which it extracts from the shell by its specially adapted habits and bill 
(page 426). The Dovekic feeds upon crustaceans floating on the sur- 
face of the open sea; in times of rough weather, the floating life goes 
deeper into the water and Dovekies go hungry. The llarpy Eagle 
of Tropical America preys upon the upside-down sloth, and an 
Eagle in the Philippines preys upon monkeys. Yet in the course of a 
year, a variety of birds may feed in a single tree or bush, each to its 
own taste: the Ruffed Grouse on buds in winter, New World War- 
blers upon leaf-eating insects in summer, Hairy Woodpeckers upon 
wood-borers the year around, Black-capped Chickadees upon insects 
hiding in the bark, Yellow-bellied Sapsuckers upon sap, Cedar 
Waxwings upon the fruit, and perhaps a Ruby-throated 1 lumming- 
bird upon the nectar of the flowers. Some of these birds may be 
designated as insectivorous, but others cannot be confined to any 
fixed listing. Some may be rather rigidly limited in their dietary and 
others can choose rather more freely. 

Methods of Feeding. Because the structure and physical nature 
of the bird relates so intimately to its feeding habits, the methods of 
feeding are necessarily subject to these structures. Among some spe- 
cies, the food habits clearly retain much that may be interpreted as 
of ancestral character. Most groups of the fish-eaters, for example, 
have probably descended from long lines of fish-caters. But the King- 
fishers, though fish-eaters, are related not to the other fish-eaters but 
to land birds not feeding upon fish. During their long history of fish 
eating, birds have developed various ways of catching their prey. 
The Kingfisher often sits upon a perch and waits until a fish appears 


fig. 23*3. (a) Outline o\ the Broivn Pelican body when Inflated (solid 
lines) ami deflated (dashed lines). (After Frank Richardson, "Functional 
Aspects of the Pneumatic System of the California Brown Pelican" 
Condor, 41 (1939):16.) (b) Lateral and (c) frontal views of a bird to 
show expansion of body by inhalation (dashed lines) and contraction by 
exhalation (solid lines). (After A. W. Schorger, "The Deep Diving of 
the Loon and Old-Squaw and Its Mechanism" Wilson Bulletin, 59(19+1): 


before flying and plunging from the air for it. The Brown Pelican 
flies low over water, locates a fish, tips up to a stall, then plunges after 
it. The pneumaticity of its body absorbs the shock and probably 
helps it to ride high in the water (Fig. 23-3). 

True divers, as distinguished from the plungers, go under from 
the surface by their own effort, not from momentum gained in the 
descent through the air. They have considerable capacity "to expand 
and contract the body, which with lowered pneumaticity of the 

Fig. 23-4. The Black Skimmer feeds by skimming over the surface of 
the water with its lower mandible cutting the surface. The wake from 
the bill of the bird on the right shows clearly. (Photograph by Ivan R. 
Tom kins.) 

bones, gives reduced specific volume (Schorger, 1947), a necessity 
for efficient submarine life (Fig. 23-3). Careful timing by an alert 
and ingenious bird watcher along an English shore established the 
principle of time-depth ratio. If the time under water of three suc- 
cessive dives be averaged for bottom feeders, the depth of the water 
will be found roughly by allowing 20 seconds for the first fathom 
(6 feet) and 10 seconds for each additional fathom (Dewar, 1924). 
Among Coots, the rule is 10 seconds for the first and 10 seconds for 


each additional fathom. The Great Blue Heron and its relatives obtain 
fish by standing in the water and seizing them. The Black Skimmer 
passes back and forth over the water, its longer lower mandible cut- 
ting the surface, ready to seize its prey (page 43, Figs. 3-9, 23-4). 

Few birds unable to go under water by diving or plunging have 
much ability to seize fish unless from above, as in the Heron or Skim- 
mer. The Waterfowl that feed by dabbling or tipping up in shallow 
waters, such as the Mallard, Swan, or Goose, have low fish-catching 
capabilities. Yet a number of interesting fishing traits have been re- 
ported, like that of a Horned Owl waiting at a hole in the ice or of a 
bird seizing a catfish gulping air at the surface. 


Fig. 23-5. The Bronzed Grackle opens acorns by rotating them in the 
bill and pressing the shell against a ridge in the palate. This cuts the acorn 
shell and exposes the meat. (After A. W. Schorger, "The Bronzed 
Crackle's Method of Opening Acorns" Wilson Bulletin, 53(1941):238.) 

Also, other food habits of birds sometimes hinge upon one struc- 
tural possession. Among Passerine birds, hardly anything seems so 
startling to an observer as to watch a Bronzed Grackle open an acorn 
with its "buccal lathe" (Fig. 23-5). 

Choice of Chief Food. The adjustments of a bird to its food 
sources include choice of foods in sizes that it can eat, as well as foods 
that the body needs or can use. Its discrimination, however, may be 
largely between wholesome foods and others. Structural limitations, 
both upper and lower, prevent many birds many times in their lives 
from feeding upon foods otherwise suitable. The Bob-white can 
eat small acorns whole, and the meats of larger ones if a Bronzed 
Grackle or Blue Jay has opened and lost them. Structural inadequacy 
limits the Bob-white in its use of acorns but not a Grackle. A Golden 
Eagle has the power to seize and eat a marmot on a cliff, but a Shrike 
does not. An Osprey can feed upon large fish, though a Common 
Tern must be satisfied with minnows. Yet even an Osprey can make 
a mistake and sink his talons into a fish too large. 

It would hardly pay the Golden Eagle to feed upon grasshoppers 
as a regular diet, but it does prove profitable to the American Sparrow 


Falcon. The Golden Eagle can hardly afford to feed upon small ro- 
dents, but in times of abundance, he might find it profitable to feed 
on them in a sort of "mass consumption" manner. More than one 
hundred thousand primrose willow seeds were found in the stomach 
of a Mallard Duck (Henderson, 1927), but no Mallard can long sur- 
vive on such foods if it must pick them one by one. A Slate-colored 
Junco, however, could perhaps do so. The Mallard can \ise small 
seeds by eating seed heads, a form of mass consumption. Under stress 
of food shortage, however, a bird may feed upon many foods not 
suitable for sustaining life for long. 

Biologists studying the food habits of game birds oftQn classify 
winter foods as preferred, staple, emergency, and stuffing. Birds 
depend upon the staple foods for energy and heat, especially in cold 
weather, though they may not supply a balanced diet. Except for 
some animal food, probably no single food item can itself supply a 
balanced diet. 

Within the limits of its innate food choice, an animal seems to gov- 
ern its eating by the abundance of foods available. The exact kinds 
of seeds and berries eaten by a White-throated Sparrow may differ 
from area to area according to what is available. In the Lake States, 
White-throated Sparrows may feed extensively on berries of the elder 
(Sawbucns) , and on those of the yaupon (Ilex) in the winter range 
in Texas. The famed incident of the California Gulls of Great Salt 
Lake feeding upon Mormon crickets in 1848 illustrates the way 
abundance may influence food choice. For this service to hard- 
pressed pioneers, a grateful people erected a notable monument in 
1913 at Salt Lake City, Utah. 

Foods of Special Need. We know almost nothing of specific 
nutritional needs of the wild bird's body, a subject to which human 
beings pay so much attention, especially for vitamins and minerals. 
The need for grit in the gizzard forms one definite use of minerals, 
but grinding material seems always to be in suitable supply before its 
loss might be critical to the bird. Even hard seeds will serve as grind- 
ing material (page 62). Presumably the bird with a free dietary 
choice gets all the minerals that it needs, though such a concept tells 
little and assumes a lot. Animal-food eaters seem likely to get all 
the dietary necessities from the bodies of their victims. Perhaps 
many birds can synthesize some vitamins within their own bodies. 

Eggs of wild birds always seem to have sound shells, and bird 
bones usually seem well made, which gives rise to the assumption that 
calcium in sufficient quantity and form is not lacking to birds. Yet 
careful study in the wild of eggs and perhaps x-rays of birds might 
show some deficiencies. A few birds seek salt, but this habit may not 



necessarily indicate that their bodies need it for proper functioning 
(page 92). Many of the same species live where such salt is not 
known to be available. (Even access to salt in wild mammals like 
the elk appears not essential, though the animals relish it.) Birds 
especially attracted to salt are some of the Finches (Red Crossbill, 
White-winged Crossbill, Purple Finch, House Finch, Evening Gros- 
beak, Pine Siskin) and Doves (Passenger Pigeon, White-winged 
Dove, Mourning Dove, Band-tailed Pigeon). The Band-tailed Pigeon 
chooses to drink salt water at the margin of Puget Sound mud flats 
even though freshwater streams are available (Neff, 1947). 

Food Storage. Cases of actual food storage are rare among birds. 
The Blue Jay, Steller Jay, Red-headed Woodpecker, and several 
other American birds put acorns in holes and crevices. Probably this 
is true food storage. Nuthatches will take pieces of food, such as suet 
at a winter feeding station, and tuck it into crevices in the bark of 
trees. Shrikes impale insects and occasionally small birds and mice 
on thorns. But this seems mostly for convenience in feeding, because 
they have no talons, rather than an example of food storage in the 
true sense. 


Food Chain and Food Web. Because the sun is the only original 
source of biological energy available, its transfer to the bird involves 
a complicated series of channels. Plants entrap the energy by photo- 
synthesis and combine it with water and sometimes other substances 
(page 198). The energy passes on to a herbivorous feeder that stores 
it within its own body. The next transfer is to an animal that feeds 
upon the plant-eater. The transfers from thence onward are from 
an animal-feeder to another animal-feeder. To this system of energy 




-* Entrapment 


- Prey 1> Predator-Upredatorf^ Predator 

Plant _ 1 Animal ^ Animal | ^ Animal 
eater eater eater 


Maple tree 

'JT Insects 

Phytoplankton^T r*- Minnow -f-Kmgf isherfCooper Hawk 

JZooplankton I J 

Fig. 23 6. Diagrammatic representation of the food chain and an ex- 
ample in land and 'water habitats. 


transfer is given the name food chain, to each stage the name link 
(Fig. 23-6). The several food chains in an environment form a food 
web. Seldom does a food chain exceed four or five links, except for 
interpredation, the preying of predator upon predator. 

The food base involved in predator-prey relations has been given 
special attention (Errington, 1946). The listing of the % bird and 
animal life of an area by numbers shows that it varies from a large 
number of some, such as rodents and songbirds, to a few of the larger 
predators. This ecological listing as a table in order of numbers has 
been called a pyramid of mtwbers (Leopold, 1933). 

Insect Suppression. Many factors cause mortality in insects or 
prevent them from developing, including malfunctioning in the in- 
sect itself, but the most important arc believed to be the favorable and 
unfavorable aspects of the habitat. Among the immediate factors is 
climate, demonstrably the most important single one. Birds play an im- 
portant general role and often an important specific one. Birds have 
many alternate foods, both insect and noninsect, so that local exter- 
mination of any one usually still leaves them a food base. This versa- 
tility measurably increases their usefulness to man and his crops; 
through this, they tend to apply pressure upon the insect life in pro- 
portion to insect abundance. By that token, the birds stand ready to 
prey upon any and all insects. Their general tendencies measure their 
usefulness (not just their specific activities) the constant restraint 
that they place upon insect life, not just their more spectacular actions 
during outbreaks. Even so, the highly mobile bird gathers quickly in 
numbers at the scene of an outbreak. The number of birds that feed 
upon grasshoppers and locusts during an outbreak has been found 
to be very large. Two hundred two species fed upon locusts and 
their eggs during the great invasion of Rocky Mountain migratory 
locusts into the western prairies and plains from 1873 to 1876 (Hen- 
derson, 1927). The Loggerhead Shrike is one of the birds that will 
turn to such insects or feed upon them regularly. It leaves signs of its 
work by impaled insects on thorns and even on barbed wire (Fig. 
23*7). But in the highly artificial environment under the conquest 
of man, the natural checks upon insects may not be sufficient, and 
artificial control efforts may be needed also. 

The insect-feeding habits of birds long have been investigated. 
Certain birds may be the chief ones controlling some insects. The 
Woodpeckers feed upon bark beetles, and foresters in the southern 
pine belt have considered them about the only control likely to be 
effective without costing more than the value of the timber saved. A 
family of Bob-whites in a potato patch will keep it free of potato 
beetles. Observation of an infestation on a single small mountain ash 



tree in the Adirondacks showed that one Veery with a nearby nest 
completely cleared it of leaf-eating insect larvae that had defoliated 
more than 10 per cent of the tree before being found and destroyed 
by the Veery, evidently to the last one. 

Such examples have occurred innumerable times, and the impor- 
tant role of the birds has been demonstrated clearly. Yet it would 
be unrealistic to accord to birds the all-controlling role as crop pro- 

Fig. 23 7. The Loggerhead Shrike, m coitnno-n with others of Its family, 
impales its food upon thorns. 

tectors envisioned in such statements as, "If the birds were all de- 
stroyed, agriculture in the United States would instantly cease." But 
the insect-suppression value of birds to agriculture clearly runs into 
the hundreds and even thousands of millions of dollars yearly. 

Insecticides and Birds. The widespread use of insecticides, 
especially in broadcast and airplane spraying, poses yet another prob- 
lem for the birds. A low concentration of spray seems to cause little 
harm, but 5 pounds of DDT to the acre in a Maryland scrub woods 
reduced the five commonest birds by about 65 per cent; only the Chat 
was uninfluenced (Robbins and Stewart, 1949). In other studies, 
3 pounds of DDT to the acre markedly reduced the number and 


vigor of young House Wrens when applied during the hatching 
period (Mitchell et al., 1953). 

Because differences in resistance to acute and chronic poisoning 
have been noticed in experimental animals, similar conditions no doubt 
occur in wild birds also. The rapidity with which chemists turn out 
new insecticides out-distances knowledge of their influence on birds; 
it also out-distances knowledge of their influence upon tnc health 
and well-being of man himself. The hazards to human health of 
more than two dozen new insecticides (Webster, 1951) indicate 
clearly a danger to birds in the wild as well. 

Birds and Plant Distribution. That birds can serve as the distrib- 
uting agents for many plants seems demonstrated, though how many 
plants and which birds are less definitely known (Henderson, 1927). 
Many seeds have been tested for viability after passing through the 
alimentary tract of birds, even of granivorous ones, and a surprising 
number germinate. Throughout the range of red cedar (Jwiipems), 
yaupon (Ilex), grape (Vitis), nightshade (Solatium) and Poison Ivy 
(Rims) , for example, the seeds passed in the droppings of birds sprout 
under isolated trees, along fence rows, and under telephone wires 
where only an animal like a bird could have deposited them. 

Few plants depend directly upon birds for seed dissemination, but 
the various mistletoes seem to do so. The berries are eaten by the 
birds, which digest the covering and pass the seeds in defecation, now 
with a viscous coating that sticks to anything touched, especially the 
limbs upon which birds perch. The Cedar Waxwing in the southern 
states and several birds of Australia, Mexico, and the West Indies 
have been reported to be distributors of mistletoe (Henderson, 1927; 
Sutton, 1951). The heavy seeds of trees like the oaks would not be 
able to move up steep slopes to replace trees higher up, and producers 
of such heavy seeds need the services of "uphill planters," even though 
only once in centuries would their services be absolutely essential in 
planting a seed to replace a tree (Grinnell, 1936). 

Birds have been reported to disseminate various blights, but this 
seems of little importance. Spores of the chestnut blight, for example, 
were found upon the plumage of nineteen species of birds, the 

freatest abundance being upon the Downy Woodpecker (Heald and 
tudhalter, 1914). But this influence compared to that of wind was 
deemed not very important except perhaps for "spot infections." 

A few plants, chiefly tropical, depend upon birds for pollination 
(hence, they are termed ormthophillus) , primarily by Hummingbirds. 
The bird feeding at a flower brushes pollen upon its plumage, which 
then may be transferred to another plant or to other flowers of the 
same plant (Fig. 23-8). 


The enormous quantity of weed seed eaten by birds has long 
served as an illustration of a service to agriculture. The total eaten 
throughout the farming areas of the world runs into astronomical 
figures. Each seed eaten in a farm field is a seed not likely to sprout 
later, though some, nevertheless, will pass through the bird in a viable 


Fig. 23 "8. Hiinmringbird feeding [row a floivcr (Ccntropogon cordi- 
folius). During the early flowering stage (a) pollen from the antlers gets 
on the bird's crown; the pollen rubs off (b) on to the stigwas of flowers in 
a later flowering stage. (After Helmuth O. Wagner, Food and Feeding 
Habits of Mexican Hummingbirds" Wilson Bulletin, 5 8 (1 946) :6 9-9 3.) " 

Predation Upon Rodents and Other Mammals. The good serv- 
ices of Hawks, Owls, and other predatory birds as controllers of 
rodents goes rather unnoticed compared to the publicity attending a 
single event considered by some a "misdeed" (sec also Chapter 22). 
In general, a Hawk or Owl preying upon rodents should be counted 
as worth no less to a farmer than a hen in the Chicken run. Particu- 
larly is this true of the western lands where rodents feed extensively 
upon forage useful to livestock. A Short-eared Owl of the British 
Isles would probably eat between 95 and 142 pounds of field mice in 
a year (Chitty, 1938). Comparable service should be expected else- 
where in the world. 

The Great Horned Owl is one of the few winged predators feeding 
at night physically capable of holding feral house cats in check. It 
also makes it well nigh impossible for brown rats to live away from 
buildings in the Northern winter. An extremely interesting and much 
cited incident indicates the inherent value of the Owl in rodent con- 
trol. Cats locked up overnight in the storage cellar of a brewery fled 
in terror when the doors were opened next morning. But an Owl 
similarly locked up killed nine rats the first night and soon cleaned 
out the rest (Henderson, 1927). Incident upon incident have been 
related and many more can be to illustrate the point: enter Hawk 
or Owl, exit mouse or rat. 




Fig. 23 9. Screening prevents birds from perching on buildi?jgs or nest- 
ing in ventilators. (By permission frow Practice of Wildlife Conserva- 
tion, by Leonard W. Wing, p. 308. Copyright, 1951, John Wiley & Sons, 
Inc., New York.) 


Bird Damage. A biological axiom holds that animals living beyond 
their native range or in ranges invaded and seriously degraded by man 
may at times conflict with man's preferential interests. The preying 
by predators upon game birds, game mammals, and game fish in 
America cannot correctly be classified as a ^damage," because wild 
animals are not the property of the individual nor are they the sub- 
jects of commercial or private use except as a privilege from the 

Red-winged Blackbirds have been found to injure corn and other 
grain crops; careful studies have shown that acetylene exploders will 
protect 10 acres of corn while eighteen half-yard square cloth dangles 
will protect about an acre (Cardinell and Hayne, 1945). Acetylene 
exploders have also been used successfully to protect orchards from 
many birds. Flashing bottles and tin or aluminum sheets have worked 
well; whirling or twisting shiny streamers have proved better. All 
seem to be as good as the traditional scarecrow and sometimes much 

The largest popular complaint, however, is directed at birds about 
buildings and city trees, chiefly against those European immigrants: 
the Pigeon, Starling, and House Sparrow. The Starling has proved 
an especial pest in a number of cities where the birds roost on the 
ledges and projections of buildings and in trees. Its role as a pest 
should be reason enough for all the strong aversion and controls 
against importing or transplanting any other animal. Various methods 
have been tried to evict them, often at great cost scarecrows, Owl 
cutouts, lights, shakers, gas, poison, shooting, fireworks, water hoses, 
nets. Recorded fright calls played back near the roosts proved 
effective in evicting Starlings (Fings and Jumbcr, 1954). But the most 
permanent results come from preventions: filling in or sloping ledges, 
plugging holes, and screening perching places (Fig. 23-9). 


Of all the several thousand species of birds in the world, barely a 
handful have been used by men as domestic stock. For practical pur- 
poses, they are here listed in three groups: (1) captives, (2) slaves, 
and (3) domestic ones. To be truly domesticated, an animal must be 
broken completely to man's will, psychologically as well as physically. 
This has not happened often. In addition to domestication, some 
commercial use may be made of wild birds, chiefly through com- 
mercial products. 

Bird Captives. Caged birds, popular with some people and un- 
popular with others, include a wide variety. But the largest number 


come from three groups: (a) Parrots, Lories, Macaws, and Cockatoos, 
(b) Starlings and Mynahs, and (c) Canaries, belonging respectively 
to the families Psittacidae, Sturnidae, and Fringillidae. Many wild 
birds have been made captive for their ability as songsters (Canary, 
Mynah), as imitators of human sounds (Parrots, Mynah), and as 
objects of curiosity (Parrots, Lovebirds, Macaws, Cockatoos). 
Canaries inhabit the Canary and adjacent islands (whence thf name). 
People raise them by the thousands throughout the world in various 
"breeds." Mynahs inhabit much of the Old World, but those used 
as cage birds live wild in India and adjacent regions. The so-called 
"Talking Mynahs" arc usually captive Hill-Mynahs. 

Several members of the Parrot group appear in captivity from time 
to time, some because they can crudely imitate human speech, others 
because of their bi/arre looks or curious habits. The ones chiefly on 
the market are the Australian and African Lovebirds, the Gray Parrot, 
Green Parrot, Yellow-headed Parrot, and Macaw of the Neotropical 
realm, and the Cockatoos of the Australian region. But any highly 
colored members of the family may be looked for in captivity, espe- 
cially in their native range. From time to time in various parts of the 
world, other caged birds may be found. 

The Ostrich, athough not strictly a caged bird, has been raised on 
farms for some years, the first during modern times in the middle of 
the nineteenth century. Hence, raising Ostriches for their plumes may 
be a recent use of birds. 

The Peafowl, also not strictly a caged bird, lives wild in Ceylon 
and India; the captive stock came into the western world at some 
ancient time. The principal use of the Peafowl is as an attraction in 
parks and about houses. 

Bird Slaves. A "slave bird," as distinguished from captives, is one 
that is forced to "serve" men. The Falcons, t lawks, and Eagles used 
in falconry are such slaves to the falconer. In earlier England, belts 
were drawn about the neck of tamed Cormorants to prevent them 
from swallowing fish. When turned loose in the water, they caught 
fish but could not swallow them. Because of training, they came to 
their keeper who by manipulation of the throat, forced removal of 
the fish. In parts of the Orient, Cormorants serve likewise. In some 
Pacific Islands, Man-of-war Birds have been used to carry messages 
somewhat as Pigeons have been used elsewhere. Few other slave birds 
have been used except as decoys (e.g., the "stool pigeon" of Passenger 
Pigeon days). 

Major Domesticated Birds. Only four birds can be considered 
of major commercial importance: Chicken, Turkey, Goose, and Duck. 


Primitive man domesticated all of them in some unrecorded time 
long ago. 

Chicken. The Domestic Chicken developed from the Jungle Fowl 
of Southeast Asia and adjacent islands. It seems to have come into 
the western world, probably through Persia, at least by B.C. 500. 
Many varieties have been developed, some of which antedate its com- 
ing into the West; of this number, about a score arc of importance. 
In its native life, the Jungle Fowl seasonally lays in sets of perhaps a 
dozen eggs. Some authorities say that it may be monogamous, others 

Turkey. The Conquistadors found the natives of Mexico raising 
a large gallinaceous bird that they took back to Spain with them. It 
soon acquired a name, presumably from its call or the fancied resem- 
blance of its head to the fez worn then by the Turks. Later this type 
of Turkey was brought back across the ocean. The domestic Turkey, 
however, comes not from the Wild Turkey that lived in New Fng- 
land, greeted the Pilgrims, and graced their Thanksgiving table; it 
comes from those domesticated by the Indians of Mexico, who, along 
with their fellows of Central and South America, appear to have an 
aptitude for taming wild birds and mammals. 

Goose. Though we cannot be exactly certain of the species that 
served in ancient times as the source of our domestic birds, the com- 
mon Graylag Goose of Europe appears to be the ancestor of most 
breeds. The Graylag ranges widely over the Old World and breeds 
over much of the northern part. It doubtless was domesticated or 
kept half-tamed by early man, probably prehistoric, in the same way 
that American Geese have been kept in captivity in the past. (It must 
be remembered, however, that the laws and regulations of Canada and 
the United States discourage the keeping of native birds in captivity.) 

Domestic Geese have changed surprisingly little from their wild 
progenitors. A number of different breeds have been developed; the 
most strikingly different from the Graylag are the white ones. Like 
most domesticated animals, the Goose has increased in size and lost 
much of its inherent wildness. But like some others, its breeding sea- 
son is still substantially that of its wild ancestor. 

Duck. The several breeds of the common Duck are all descended 
from the common wild Mallard of Europe, a bird closely related to 
the New World Mallard. Many of the breeds (e.g., Indian Runner, 
Pekin) have departed rather greatly from the Mallard type. The exact 
time of domestication is unknown, though there is little doubt that 
primitive man did the domesticating. 

Minor Domesticated Birds. Four birds may be listed as minor 
domestic ones for the simple reason that raising them in domestica- 


tion is infrequent and they are not very important commercially or 
the industry has not become concentrated anywhere as compared 
with the major ones. 

Among these is the Sivan, descended from the Old World Mute 
Swan. Just as in the case of other, domestic birds, the time of domesti- 
cation is in doubt. Though once raised for the table, Swans today are 
kept chiefly in parks and on estates. Occasionally tamed Swans other 
than the Mute appear as captives. When Swan-raising flourished on 
the Thames, in order to distinguish the birds, distinctive marks of 
ownership were required upon the bills, which "swan-marks" were 
recorded by the Royal Swanherd. (In like manner, cattle on the 
western ranges today carry identifying brands, which are registered 
in an official brand register and checked by brand inspectors.) 

The Muscovy Duck was domesticated by the Indians of Central 
and South America at some unknown time in the past. (Its range in 
the wild reaches almost to the Rio Grande in Mexico.) Though it is 
raised in domestication, the number is not large compared with other 

The Guinea Fowl of Africa seems to have been domesticated by 
natives of the Guinea Coast of Africa and probably introduced into 
the civilized world by the Greeks or Romans, but certainly before the 
fifteenth century. Guinea Fowls are raised mostly as curiosities, 
though some are raised as a delicacy. They have changed little by 

All our breeds of the Pigeon or Dove probably have developed 
from the common Rock Dove of the Old World. Yet its ancestral 
relationship and home are somewhat in doubt. Although it is not cer- 
tain, it seems unlikely that much admixture of blood from other 
species has occurred (page 398). Pigeons arc raised as curiosities, for 
the table, for carrying messages, for racing, and for experimental use. 
There are some two hundred different breeds. 

Commercial Products. No commercial uses any longer may be 
made of wild birds in the United States and Canada, though elsewhere 
commercial uses are practiced. The protective tradition of the United 
States and Canada may in time become established in other regions of 
the world. 

Probably the best-known commercial use of all, the guano in- 
dustry, is not a commercial use of the bird itself but of its excrement. 
Bird guano accumulates in arid regions where birds nest in colonies 
(Hutchinson, 1950). Rich, cold, upwelling ocean waters supply 
quantities of marine life. The bird islands of Peru (Murphy, 1925) 
particularly have been noted for guano, and the industry has been 
organized and controlled on a management basis. Many chemicals 


occur in guano, but the principal ones for fertilizer are nitrates that 
may reach 26 per cent and phosphates that may reach 18 per cent but 
in inverse relations with each other. In actual practice, the guano 
birds bring back to shore in a relatively small way some of the fertility 
washed to sea by the rivers. 

In South America lives a most interesting bird, the Guacharo, 
called also the Oil-bird (Steatornithidae), one of the few night- 
feeding fruit-eaters in the world. When the very fat young are about 
two weeks old, natives gather them from their nests (usually in caves) 
and render them for a colorless cooking and illuminating oil. (The 
birds living in caves are believed to use acoustical orientation.) The 
Great Auk gave oil when its body was tried out, but it was pursued 
also for eggs, meat, and skins. The Penguins too have from time to 
time suffered pursuit for oil, the tried-out carcasses serving sometimes 
to heat up the rendering kettles for others. 

In some parts of the world, though not in America, people raid 
sea bird colonies for eggs to eat (Cott, 1953). Often the colonies are 
systematically "egged" from the beginning of the season on, which 
gives the eggers some assurance of relatively fresh eggs. 

The feather trade for millinery purposes made a number of birds 
extinct or nearly so, the Egrets along with others. The cutting off of 
the American market by prohibition of importation of millinery 
feathers (Tariff Act of 1913) stopped most of the destruction, though 
it has continued in out-of-the-way places and in a few backward 
areas. Any source of legal feathers (other than from poultry) is 
bound to be small (such as Ostrich farms). 

Few bird feathers have been used for clothing except in trimming, 
though Indian medicine men and others used them for ceremonial 
costumes and decorations. Montezuma's headdress (perhaps actually 
a cloak) was of this nature. The cloak of King Kamehameha of 
Hawaii was made of feathers from the Mamo. Eskimos may use skins 
of Eider Ducks for making clothing. 

In some lands bordering upon the North Sea, North Atlantic, and 
Arctic Ocean, people collect the down from Eider Ducks (mostly 
from the nests) and sell it, the eider down of commerce. Because to 
a people living a marginal existence it forms an item 6f income worth 
going after, considerable protection accrues to the nesting birds that 
more may nest and that more down may be collected. 

The ancient and primitive use of birds as medical nostrums surely 
should not go unnoticed. The meat of the English Robin and Hedge 
Sparrow, for example, was deemed effective in the Middle Ages for 
elimination of stones and other obstructions of the urinary system 
(Lack, 1946). When made into a powder by drying and burning, it 


was considered to be as effective as eating of the meat itself, which 
seems highly probable. Even some bird dung was thought to have 
medicinal properties. 


* ALLEN, ARTHUR A., The Book of Bird Life. New York: D. Van Nostrand Co., 

Inc., 1930. 
AMERICAN ORNITHOLOGISTS' UNION, Fifty Years' Progress of American Ornithology , 

1883-1933. Lancaster, Pa.: American Ornithologists' Union, 1933. 
ERRINGTON, PAUL L., "The Pellet Analysis Method of Raptor Food Habits Study," 

Condor, 32 (1930): 292-296. 
*FORBUSH, E. H., Useful Birds and Their Protection. Boston: Massachusetts State 

Board of Agriculture, 1927. 
*HENDERSON, JUNIUS, The Practical Value of Birds. New York: The Macmillan Co., 

KALMBACH, E. R., "Field Observation in Economic Ornithology," Wilson Bulletin, 

46(1934): 73-90. 

PICKENS, A. L., "Bird Pollination Problems in California," Condor, 31(1929):229-232. 
*WING, LEONARD W., Practice of Wildlife Conservation. New York: John Wiley & 

Sons, Inc., 1951. 

*YocoM, CHARLES, Waterfowl and Their Food Plants in Washington. Seattle: Uni- 
versity of Washington Press. 1951. 


Bird Study Afield 


Living birds in the field attract the interest of many people because 
of their beauty and sprightliness; the outdoor appeal inherent in them 
attracts many more. Their evident independence and their freedom 
of movement, freedom to come and go, evoke both sympathetic 
understanding and twinges of envy in still others. No one reason fits 
all people or even the same person at all times. But it seems certain 
that few ornithologists are so lacking in sentiment as to look upon 
birds as merely the sources of cold, scientific data. The simple fact 
remains with us always: people study birds afield because they like 
birds and the outdoors in which they live. 

The fact that birds stir the enthusiasm makes them especially fit 
subjects for the rewarding avocation of bird study. Arousing enthu- 
siasm in the course of scientific research of a more profound nature 
undoubtedly enhances the value of the work. Objects of research 
need not be dry and unattractive in order that the science in their 
study be profound and objective. In actual fact, science gains when 
scientists study their subjects with the zest of the avocational worker. 
The more thrilling the study, the more stimulated the mind, and the 
more stimulated the mind, the greater the likelihood that it will con- 
tribute more to the advancement of science. Many of the best orni- 
thologists started out as avocational bird students, an outlook that left 
few in their lifetimes. Many another scientist received his scientific 
baptism and scholarly initiation in bird study. 

As in any science, people vary in their approach to ornithology. 
Generally speaking, studies may be divided into two kinds: the study 
of problems (the common kind) and the study of principles (the un- 



common kind). Problem research deals with the specific; principles 
research deals with the general For successful fruition, the former 
requires careful pursuit of the problem, especially in its details, step 
by step to its ultimate solution or ultimate contribution of informa- 
tion. The latter requires an ability to compress details, in many fields 
when need be, in order to focus upon the principles involved. Because 
of the wide difference of outlook, conflicts arise, though nbt neces- 
sarily so if the difference in emphasis is kept in view. 

Observation and Scientific Method. Though we read frequently 
of "new approach," "modern outlook," "modern research," and many 
similar expressions as though indicating that the methods of earlier 
ornithologists are archaic, the observation and experimental methods 
of bird study have remained surprisingly similar for many decades. 
The demands of accuracy, care, and precision now differ little from 
those of yesterday. Ornithologists of each succeeding year have a 
greater background of knowledge or more refined techniques and can 
interpret more broadly than could earlier bird students. The ornitho- 
logical science of today, yesterday, and tomorrow differ primarily on 
which rung of the ladder they stand, not which ladder they use. 

The basis of all science is observation, be it an observation under 
natural conditions or one under experimental conditions. No exact- 
ness not found in observations afield also is inherent in an observa- 
tion of events under the most controlled of laboratory conditions. 
In the laboratory an observation and its interpretation are likely to be 
more complete than in the field because the significance of events 
concerned may be more completely understood. But the number of 
observations improves the validity of any interpretation. Conclusions 
once believed established may grow archaic when the future has un- 
rolled new knowledge. Data once thought complete may prove in- 
complete as time gathers more information. But an observation of 
facts once correctly made will remain corr'ect, though our understand- 
ing of it may vary. 

Field observation has been the source of nearly all our informa- 
tion on the living bird and has been the basis for developing much of 
that knowledge gained from preserved or experimental material. Field 
observation remains the chief source of bird knowledge and no doubt 
will in the future as it has in the past supplemented though it may 
be by laboratory study. Most field data results from direct observation 
of birds and their environment, even though some disturbance may 
arise from the presence of an observer. This soon passes and birds live 
as though no observer were about. Suitable use of binoculars, blinds, 
and observational skills eliminates all or most all of the disturbance 
arising from the observer's presence. Indirect observation is the read- 



ing of sign that tells of bird presence, its activity, and its way of life. 
Sign may be tracks, droppings, marks, feathers, or any one of an 
infinite number of things that show the action of the bird. 

Measurements Afield. The development of techniques for meas- 
uring both quantitative and qualitative data marks an event in orni- 
thological science that has given precise measurement for use in field 
records. Among the most important of these have been methods for 
measuring territory, behavior, and abundance now added to previous 
measurements, such as of time and space. The development of systems 




Mountain Chickadee Pigmy Nuthatch 

Fig. 24 I . The live actions of birds way be measured and the measure- 
ments evaluated by other observers, who way also compare the meas- 
urements 'with others, (a) Leg position of Scaup Duck compared 'with 
Mallard. (After Jean Delacour and Ernst Mayr, "The Family Anatidge" 
Wilson Bulletin, 51(1945):24.) (b) The clinging posture on a tree trunk 
differs in the Mountain Chickadee from that of the Pygmy Nuthatch as 
shown in tracings from photographs of birds at same site. (After Frank 
Richardson, Adaptive Modifications for Tree Trunk Foraging in Birds, 
University of California Publications in Zoology, 46(l942):317-368.) 

of measure provides field ornithology with a tool as important in its 
sphere as dissection in anatomy. Two observers measuring courtship 
performances of the same or of different species can gather com- 
parable and completely reliable data. Of this reliability there need 
no longer be further question. 

The relative stance of the Diving and Dabbling Ducks, for ex- 
ample, can be measured through descriptions and drawings (Fig. 
24-1). The adaptive nature of posture in clinging to tree trunks can 
be measured for birds like the Chickadee and Nuthatch (Fig. 24-1). 
Measurements of living action add to the advancement of ornithology, 
for they may be evaluated and compared with confidence in their 


exactness. The skill of the observer in the field (as in the laboratory 
or library also) limits ornithological research often before methods of 
measurement limit it. But the development of new methods of meas- 
urement goes on continuously. Field ornithology has contributed so 
much to the biology of living things that in their capacity to observe 
and measure in the field, bird students very justly may take great 

Role of the Avocational Scientist. It may be said with little fear 
of refutation that except for such sciences as chemistry and medicine, 
ornithology leads the scientific field in the gathering and understand- 
ing of knowledge. The reason for its position in advance of so many 
other sciences rests largely on its great supply of avocational scientists, 
coupled with the innate personal interest that draws people into bird 
study. A large proportion of the references and suggested reading 
titles in this book have been written by nonprofessionals, which 
testifies to the effectiveness of nonpaid, avocational scientists in orni- 
thology. (Actually, most of the ornithology of the English-speaking 
world is done by "amateurs/') 

For bird study, the need may be simple a bird in the back yard, 
binoculars about the neck, a notebook in one hand, a pencil in the 
other. Bird study may involve long trips, journeys to foreign lands, 
remote places, rigorous climates. It may involve early rising, late re- 
tiring, odd hours, long days. It may take one afield in rain, heat, cold, 
wind, snow. The way may be through swamps or over tundras amidst 
clouds of mosquitoes; it may be through desert with hot sun, deep 
thirst. All this and more too, the avocational scientist calls fun works 
longer, harder, more carefully at it than otherwise. For this, orni- 
thology is the gainer and to it science owes much. The charm of 
birds adds measurably to the advancement of science by stimulating 
the study of birds. 

Observational Techniques. No amount of description can tell 
one as much about how to observe as the actual practice of the art. 
Most field guides include instructions on observing (e.g., Pough, 
1946), books on bird watching are available (e.g., Nicholson, 1932; 
Hickey, 1943; Fisher, 1951), essays on bird study may be consulted 
(e.g., Griscom, 1945), and textbooks contain techniques (e.g., Wing, 

Direct observation requires skill in seeing things (which can be 
developed with practice) and care in interpreting their meaning. The 
latter comes with experience and knowledge, especially in knowing 
what others have discovered. This book, it is hoped, will help in pro- 
viding some background knowledge, though it must be supplemented 


by practice in the field. Reading of sign needs a far more practiced 
skill than does direct observation. An observer of nesting, even from 
a blind, cannot watch events every minute from start to end of the 
nesting period unless he teams up with others. A nest destroyed often 
bears characteristic work of the destroyer. An empty nest may be 
vacant because eggs have hatched, have not been laid as yet, or have 
disappeared. Sitting in a blind watching a Cowbird remove an egg 
from an Ovenbird nest (e.g., I bum, 1941) is a direct method of ob- 
servation. But determining from the way the eggs were eaten that a 
skunk did the job is an indirect method of observation through the 
use of interpretation. 

Scientific Method. Field observation under natural (and some- 
times experimental) conditions uses both direct and indirect observa- 
tion to amass data for analysis. From these data by interpretation 
through inductive reasoning, scientists arrive at conclusions. The 
"scientific method" consists of carefully collecting facts, testing their 
validity the meanwhile, analyzing and interpreting the data, and draw- 
ing logical conclusions. No one should suppose for a moment that 
conclusions so drawn are immutable. They must always be considered 
as based on the material of the moment; they may be changed with 
time in some cases, they may not in others. Thus, when Aristotle said 
that large birds migrated across the Mediterranean to Africa, he stated 
a conclusion unchanged after twenty and more centuries. But when 
he repeated the idea that small birds "hitch-hiked" across on larger 
birds, he stated a conclusion long since rejected. The acceptance of 
a conclusion in any field of science varies widely. It depends upon its 
timeliness, adequacy of data, reputation of its author, strength of 
contrary opinion, and vigor of its presentation. That many papers 
unearth and support the neglected work of others shows how even 
sound scientific work may not be recognized at once for its true 

Conclusions often develop into generalizations, which vary in the 
degree of establishment and acceptance. Though much variation in 
meaning and differences of opinion make for overlapping and even 
use of different terms for the same general thing, scientists use such 
terms as postulate, hypothesis, theory, rule, principle, and law for 
general conclusions. Thus, the Eiogenetic Laiv and Theory of Re- 
capitulation mean the same generalization. The Bergtnami Rule is 
sometimes called the Bergwann Principle or even Bergwann Law. 
Postulates, hypotheses, and theories are generalizations of a lower 
order of acceptance; they are temporary conclusions that may advance 
later to the "generally accepted" and "proved" category. But many 
are discarded. 



Field Records and Notes. Bird students keep records in a variety 
of ways, all suited more or less to the experience and needs of the 
user. No one method has all the advantages and much can be said for 
each of the different ways in vogue. By and large, personal con- 
venience, personal preference, and the way one started determine the 
method of keeping records. Most bird students keep journals of some 
sort into which they enter notations and observations on matters of 
interest. (Fortunately, bird students or their heirs tend to provide 
for permanent preservation in museums, libraries, and scientific 
archives.) Commonly, notebooks take the form of journals in literary 
form. A few ornithologists use small notebooks and transfer their 
records to a card file or other systematized record on a species basis. 
Others prefer a small notebook for brief notes at the moment of 
observation, which they later expand when transferring to the journal, 
generally at night. But still others carry the journal or pages from 
it afield and write down completely the observation at the time (which 
is to be preferred). A note so written leaves out less, but one written 
at leisure may be more exhaustive though largely left to memory. 
But it should be emphasized that memory is fleeting and after a few 
hours may be inaccurate. 

Little preference need enter into selection of paper other than that 
it be a good permanent, rag paper. Some people prefer buff color, but 
white is satisfactory, for one may turn his back to the sun and thereby 
reduce glare. Ink tends to smear in wet weather; it may fade unless a 
carbon ink is used; but few carbon inks work well in most fountain 
pens. Soft pencils may smudge if used on both sides of a page and 
sometimes do so when used on one side, but dipping the page in water 
and drying it out reduces smudging. Hard pencils ordinarily are use- 
able, though they may not write very black under some conditions. 

Preference also determines the size of the notebook or card file. 
But in general, standard business sizes should be preferred. Those 8 1 / 2 
by 1 1 inches serve well but prove ungainly in the field. One-half this 
size (5J/2 x ^Vi) seems best, though many people prefer an inter- 
mediate size. For card files, the 3 by 5 and 4 by 6 usually serve for 
most purposes, but larger sizes have their uses. 

Migration, nesting, and similar data lend themselves very well to 
record books similar to the account journals of bookkeepers. A hori- 
zontal line may be used for each species and a vertical column for 
each day or week. These may be used also for keeping a daily account 
of birds seen. In order to avoid the repeated writing of names, short 


sheets may be used as inserts, or one may use a large sheet nearly 
wall size. 

For repeated observations of the same kind, prepared forms ma- 
terially speed record keeping; they reduce omissions and forgetfulness 
also. When a number of different observers collect field data on the 
same kind of thing, forms materially increase efficiency afield. The 
working up of data later will be easier and more complete. For a 
scries of nest observations, an observer may with profit prepare a set 
of forms providing a space for common entries, such as nest site, nest 
size, height above the ground, distance from trunk, number of eggs, 
band numbers given to the young, and many other items. Some ob- 
servers provide for very detailed collecting of information in this 

Use of Maps. Maps have many uses and even the poorest drafts- 
man can make a useful field sketch. Maps may be made by simple 
visual estimate from a point of vantage. But more accurate ones are 
made by pacing or using a plane table (Raisz, 1948). Maps may be 
made from aerial photographs now available in the courthouses of 
nearly all the settled parts of the United States and Canada. Plat maps, 
road maps, topographic maps, land-use maps, soil maps, and some spe- 
cial kinds may be found for various areas. Many such available maps 
will serve as a convenient base for tracing a working map; some may 
be used directly. A small supply of maps can be made by tracing 
over carbon paper, but a stationery store will run off a supply by 
duplication at very little cost. 

Field Identification. The facility for identifying birds afield has 
been materially aided by the several field guides available; these de- 
scribe the respective means for identifying any bird in the United 
States and Canada (see Suggested Reading). Similar guides for other 
parts of the world are a much-needed item. But no field guide will 
substitute for practice and no field guide will convey some of the 
subtle differences often used by more experienced observers. Birds 
clinging to a tree trunk as shown in Fig. 24-1 readily indicate Chicka- 
dee or Nuthatch to one familiar with them. Without too much diffi- 
culty, an observer in the western forests could tell fairly easily what 
species. The fact that the Raven has a rather hawklike flight gait 
readily distinguishes it from a Crow to many observers. The quick 
flaps and bobbed look of the Black Vulture are unlike those of the 
Turkey Vulture. To some, the Pelicans can be identified by their 
peculiar look when sailing, as though "sitting through the air." Many 
of these subtle marks can be described, but others must be learned by 
the observer himself. 


Going Afield. There is no such thing as a "best time" for going 
afield for general bird study. Each purpose has its own time. Like- 
wise, there is no best place, for each purpose also has its own profitable 
places. If one wishes to see birds at the height of activity and anima- 
tion, for example, probably the best time would be in the spring 
shortly after sunrise. At such a time, birds will be active, the males 
will be singing, often on exposed perches. And no greater thrill can 
come to the bird watcher than the springtime with birds singing. 

Where to go depends upon the birds wanted and the season of the 
year. An observer with a small amount of time will find many suitable 
places near and far, some of which have been listed for convenience 
(Pettingill, 1951, 1953). 

Few ornithologists study birds at night, but nighttime has many 
attractions, though staying up without sleep is hardly one of them. 
The singing of nocturnal birds is definitely a subject for night study. 
Many Shorcbirds feed at night, especially in the moonlight. Many 
ground birds can be caught and banded at night by headlighting and 
catching with clap-nets. Bird eyes usually reflect as brilliant orange- 
red at night, often about the color of a live coal. This eye shine can 
be used in night study (Van Rossem, 1927). But the exact color 
varies with the intensity of light and the angle of vision. Among those 
shining red are Killdeer, Woodcock, Barred Owl, Whip-poor-will, 
Nighthawk, Chuck-willVwidow, and Poor-will. Night-roosting by 
ground and tree birds alike is little known, and some suggestions for 
study have been made (Moore, 1945). 

Bird students regularly keep lists, such as by trip, day, or habitat 
type. For convenience, some prepare printed check-list cards on 
which to keep records. Others prefer to write the names of birds as 
seen and then to enter the numbers. Printed cards have the advantage 
of stimulating the mind, so that fewer birds are likely to be over- 
looked. For increased usefulness, field record cards may include items 
of weather, habitat, and time. The usual ones recorded include many 
of the following and sometimes other ones also: 

Date Humidity 

Hours afield Rain 

Hour of observation Visibility 

Place Barometer 

Temperature Altitude 

Wind and direction Cover type 

Cloudiness Vegetation conditions 

Few daily lists of more than one hundred birds can be amassed by 
an individual except by great effort in favored localities or during the 
migration season. A few dozen species is the more usual number. 


A yearly list of two hundred birds in one locality is good. One hun- 
dred fifty is the more usual number for regions away from water. But 
some remarkable lists have been made by observers having the op- 
portunity to travel and the initiative to get out with the birds" and hunt 
for them. Life lists may run several times the seasonal list; it depends 
largely upon how much the observer travels. 

Optical Instruments. The chief optical instrument used in bird 
study is the prism binocular (Fig. 24-2). It is to the field observer 
what the microscope is to the laboratory technician (and it is just as 

Bokelite eyepiece cop* retain their Individual focusing or AH air-gloss surfaces art 

smooth black appearance and ore central focusing eye- Balcote anti.refloction 

n cold weather. ^ pieces are obtainable. coated, for increased light 

transmission and greatly 
'mproved image contrail. 

Plastic finish over body (moroc. 

co groin), weather-proof, wear. 

proof, attractive, furnishes a riimt are securely 

good grip. Cannot peel off eld in position. 

Strop eyelets cast 
integral with bod/ 

Front is integral with Precision hinge and Nine ultra-precise optical units. large objectives collect 

body - eliminates one diamond-turned axle made from six kinds of B&L glass, maximum light ... are of 

nd cover strong and maintain alignment. are required in each side of the extreme value in dork 

wtatheMight model illustrated weather and twilight. 

Fig. 24 2. Cut-away view showing parts of a sturdy, well-made, wide- 
field prism binocular suitable for bird study. (Bauscb & Lomb Optical 

reliable). Much has been written of binoculars (especially by adver- 
tising writers) but other than for coated lenses, no significant optical 
improvements have been made since the turn of the century. Improve- 
ments have been chiefly mechanical, especially in the use of light 
alloys, as well as the perfection of coated lenses. The commonly used 
bird glasses are of six, seven, eight, nine, and ten powers of magnifi- 
cation, denoted respectively as 6X, 7X, 8X, 9X, and 10X. The 
objective lens determines the amount of light that enters the binocular, 
and the "light-gathering power" is usually given as the square of the 
quotient obtained by dividing the diameter of the objective lens in 
millimeters by the magnification. The construction of inferior binocu- 


lars, however, may not permit utilization of all the light entering the 
objective lens. A 6 X 30 glass has a light-gathering power of twenty- 
five as does also a 7 X 35, or an 8 X 40. No glass of less light-gathering 
power than twenty-five should ordinarily be purchased for bird study. 
For especially clear observation in deep woods, on dull days, or at 
twilight, binoculars with large objective lenses are favored. The 

7 X 50 is the most commonly used one of this kind, though 6 X 42, 

8 X 56, and others are available and are excellent instruments. Manu- 
facturers sometimes designate such a large objective lens binocular as 
a "night glass." All such glasses are heavy to carry, but their superior 
qualities offset this for many observers. On a clear day, however, 
the limit of light is in the human eye itself, and all clear binoculars 
of good quality are about equally suitable in sunlight. 

The construction determines the field of view (not the relative 
size of the objective lens), so that a 6 X 30 glass may have a wider 
field than a 7 X 50. Manufacturers give the field as the diameter of 
the observation view at 1 ,000 yards. (In order to be more impressive, 
advertising writers prefer to specify this distance in feet rather than 
yards, e.g., 450 feet at 1,000 yards instead of 150 yards at 1,000 yards.) 
Wide-field binoculars have an extra lens, and are rather more expen- 
sive than regular binoculars. Since so much depends upon field of 
view for rapidly locating birds, one should get as wide a field as pos- 
sible. Because the field decreases with increase in magnification (unless 
constructed to compensate for this), low-power binoculars are easier 
to use in locating an object. They also may be held more steadily in 
the hand. The usual regular binocular has a field of view of about 
330 feet in the 8X and about 450 in the 6X. A wide-field 8X will 
have a field of about 450 feet. 

Center focusing and individual focusing eyepieces have their re- 
spective advantages and followers. For hard and long service, the 
sturdiness of the individual focusing should dictate its choice. With 
experience, it may be focused with about the same rapidity as the 
center focusing type. In practice, the observer focuses on some dis- 
tant object (which gives him "infinity" as in a camera) and notes or 
marks the reading on the barrel scales. For fully three-quarters of his 
observations, the average ornithologist need make no further adjust- 
ments, for his eyes can do it for him. For closer objects, one adjusts 
the eyepieces. By a glance at the barrel scales, the binoculars may be 
returned to the "infinity " (hence, "fixed focus") position, which 
constitutes the "ready" position for him. 

Because so many binoculars of inferior quality (mostly imported) 
have flooded the market, the bird student should tread warily in pur- 
chasing any binoculars other than those made by one of the standard, 


long-time manufacturers (Reichert and Reichert, 1951). For used 
glasses of any make and new glasses of any except the few time- 
tested ones, the bird student should depend upon competent advice (a 
rarity) before purchase. Surplus glasses of the American army or 
navy (even of World War I) are optically and structurally well 
made, though the condition of wear may render them unsuitable for 
purchase. Even so, many glasses are not suitable or convenient for 
bird study, though they may be sound instruments nevertheless. Few 
dealers in the field are competent to advise on the matters of binocu- 
lars, and the prospective purchaser of binoculars is well advised to 
seek the recommendations of qualified ornithologists. 

Spotting scopes are useful in the study of Waterf owl, birds in moun- 
tains, and for many other special purposes. A few binoculars of great 
magnification (16X to 24X) have been manufactured that arc ex- 
ceptional instruments for special studies with a tripod. They may be 
used in nesting studies, for example, at enough distance from the nest 
to omit a blind. Spotting scopes and telescopes have been sighted 
against the moon for migration studies. Radar lias been tried in flock 
studies. Infrared spotting devices have been tried for night work. A 
mirror on a rod serves as an instrument for examining nests out of 

Visual Distance. The distance at which a bird may be recognized 
varies with observers and conditions of observation. For practice in 
estimating distance, cutouts or wooden models may be perched or 
suspended by threads at known distances. Some measure of the visual 
distance in feet for good eyesight has been reported (Table 24-1). 

Table 24*1 
Visual Distance for Good Eyesight 

Species Visual Distance 

Hummingbird !t)0 ft., on wire 

Swallow 250 ft., on wire 

American Goldfinch 200 ft., flying 

American Robin 250 ft., identified by shape 

500 ft., shows as a dot 

750 ft., noticed only by the best of eyes 

American Crow Can be seen twice the distance of the Amer- 
ican Robin 

Broad-winged Hawk Recognized as Hawk at one-half mile 

Turkey Vulture 4,700 ft., recognized as a soaring bird 

Source: Harold B. Wood, "How Far Can a Bird Be Seen?" Auk, 54 (1937): 96-97. 

Photography. No particular "tricks" are necessary for most bird 
photography other than skill, patience, and care. For good work, a 


suitable camera is essential, but for an occasional picture now and 
then, many common cameras are useable. Color photographs are often 
taken in 35mm. size because of convenience and expense, but larger 
si/es take better pictures. Many people use miniature black and white 
cameras with success, but no small camera can take the quality picture 
produced by larger cameras, except under fortunate circumstances. 
Reflex cameras, particularly the Graflex with extension bellows, take 
the largest proportion of pri/e winning pictures. Telephoto lenses and 
extension bellows are essential additions to the equipment of a bird 

A small, compact camera with an extension bellows proves to be the 
most versatile, though not so handy as a miniature camera nor so likely 
as the Graflex to take superior pictures. Yet anyone who plans to 
take pictures in the field must expect to put some effort and care into 
each picture. Success in bird photography seldom comes by taking 
snapshots; effort pays off. The camera has many uses in bird study, 
such as the study of action, posture, behavior. (Fig. 24* Ib shows an 
example of the study of clinging by tree-trunk birds by means of a 
camera set to take pictures at the same spot; the outline of the birds 
has been drawn from the photographs.) The taking of flight pictures 
by high speed photography has been especially successful and offers 
opportunities for further study (see Fig. 15*7). 

Motion pictures in color have an important role in instructing 
people in bird life. Since the lives of birds are most entertaining sub- 
jects for pictures, skillfully edited motion-picture photography of 
birds in color can hardly become an exhausted art in the foreseeable 
future. Motion pictures have been used successfully in the study of 
motion and behavior, but color film may not be needed. In general, 
all the precautions, problems, and skills of still photography apply 
with equal or greater force to motion-picture photography of 

Use of Blinds. Blinds may be built of almost any material avail- 
able roofing paper, bodrds, canvas, and even cornstalks (Fig. 24*3). 
Some photographers and ornithologists use portable blinds. Blinds 
may be placed in trees, on floats, or on towers; they may be in the 
open, in the brush, or in the woods. Birds soon learn to ignore a 
blind, even though it may be only a few feet from a nest. Often the 
blind is built some distance from the object and moved closer each 
day until it is in final position. Holes in the sides give openings for 
cameras and for watching. In general, blinds may be used at any 
season of the year, and they may be of any size that is easy to build. 
Larger ones are more comfortable, which comfort increases the 
observer's efficiency. 




IfiWfeH ft " > ' ' '' - * v^'V'i ','* ",' ' 'i ^ ; v .' 


Fig. 24-3. Cornstalk blind built to photograph and study Prairie 
Chickens at a 'winter feeding area. (Photograph by Leonard W. Wing.) 

Recording Instruments. Many normal laboratory and weather 
instruments such as the kymograph, thermograph, barograph, hygro- 
graph, and anemometer have long been used in bird study. A number 
of special devices have been developed, and ingenious ornithologists 
yearly develop more. An activity recorder consisting of a drum turned 
by a clockwork, a roll of adding machine tape, a recording pencil, 
and nest contacts will keep a record of the coming and going at a nest 
box or at an open nest. Any ornithologist of modest mechanical ability 
can construct one.* More complicated instruments are the recording 
potentiometers that use thermocouples. t The development of radio- 
active trace elements and Geiger counters gives a new tool for field